Religious symbolism of the twins
Many people in the Iron Kingdoms openly worship Morrow, and many of them have a patron Ascendant, whether they are formally dedicated to that patron or not. The likenesses of these Ascendants appear prominently in paintings and statuary in the homes and courtyards of the faithful. Small tokens featuring symbols representing these Ascendants are attached to the Radiance of Morrow as a part of a dedicated priest's holy symbol, embroidered on some articles of clothing, or emblazoned upon tools, weapons, and armour.
I see the depictions of Thamar's Scions as being a bit more abstract and less-easily-identified than Morrow's Ascendants. Only Stacia is routinely depicted by her actual gender, the others varying to suit the needs of whomever created the likeness. Few are the places where the open worship of Thamar is accepted (e.g. nobody wants to do business with a merchant that worships Scion Bolis), so these figurines are often kept hidden away in secret shrines. Only extremely-influential Thamarite congregations can risk the construction of large temples and cathedrals, and even then they are hidden away and guarded. The symbols of these powerful beings are normally only carried by the very bold or very faithful, and even then are not displayed prominently.
There is a great deal of variation in the depictions of individual Scions, due to the decentralised nature of Thamar's following, but certain themes keep recurring either through divine inspiration or by simple association with the Scions' spheres of influence.
The Order of the Masque
While it is true that the Order of Keeping and the Order of the Fist are the only two publicly known monastic orders in Western Immoren, they are not entirely alone. While more Cyrissists aspire to replace their flesh with undying steel and brass, there are those who believe in transcendence of a more spiritual nature. Inspired by the contemplative orders existing in the Morrowan and Menite churches, the Order of the Masque exists as a home for those who feel called to explore the Maiden's mysteries through intuition and meditation.
Most of the Order's monasteries, what few there are, exist within larger Cyrissist communities, either inside a temple complex or close nearby. Sometimes the monasteries are mobile: one or several masters will take their students with them on the road, traveling from temple to temple to absorb teachings and insight from Cyrissist communities and the world in between.
As part of their regimen for training their bodies to overcome physical obstacles on the road to enlightenment, brothers and sisters of the Masque learn armed and unarmed combat. Weapons favored by the Order are those resembling trappings of the faith, such as small gears with sharpened teeth that function as throwing implements, and lengths of chain like those used in large clockwork engines.
Monks away from their monasteries are frequently agents of one Cyrissist sect or another out on a mission for the faith. Others take specifically to adventuring with a band of non-Cyrissist companions. Understanding the Maiden's many mysteries requires profound wisdom, the likes of which the experience acquired in the varied and exciting adventuring lifestyle can help to cultivate. As the existence of their order is a secret, they typically present themselves to their companions as very agile and self-possessed, but otherwise ordinary warriors.
Adherents of the Order of the Masque usually have the Masque of Cyriss tattooed on their body in a discreet location. They can use this as an identifying mark in a pinch, but they prefer to exchange a shared revelation of lower-level mysteries for that purpose.
General notes about IK Religion
The primary "pragmatic" reason to worship a god, other than merely the desire to belong to a faith of one's upbringing, is the reassurance that a place may be awaiting you in the afterlife within the relatively safe domain of one's patron. The afterlife as depicted by the residents of western Immoren is a cruel and largely unpleasant place, particularly for non-believers.
Those who are not faithful are expected to face tremendous unpleasantness in Urcaen, being lost in something of a cosmological wilderness surrounded by hostile and cruel beings that delight in nothing more than preying on loose souls. Whereas those who have been pious and faithful have the hope of a relatively sheltered and rewarding afterlife, should they be shepherded to their god's domain. Because of the War of Souls even those who arrive in a god's domain are not expected to experience endless "peace and tranquillity", as each god's domain requires ongoing vigilance and periodic battles to preserve the safety of the majority. The general consensus is a soul which arrives in their god's domain has a generally positive existence in the afterlife, periodically interrupted by short stints defending the borders of their god's domain or performing other useful services as required. Service to one's god and his domain is a nearly universal part of every religion which includes an afterlife in Urcaen. (Thamarites being the least focused on this aspect, preferring to imagine they will have great liberty in the hereafter, so long as they please the Dark Twin.)
This might be considered a grim outlook on the afterlife compared to some religions, but the inhabitants of western Immoren take these concepts for granted as just part of their reality. Most pious individuals anticipate being rewarded for their faith and actually look forward to serving their god more directly in the hereafter.
There is no particularly compelling reason to avoid paying at least lip-service to one or another of the gods, and most individuals, even those not particularly "spiritual" look at observing some of these formalities as "hedging their bets" for when they die. All the religions of western Immoren are generally in agreement that avoiding worshipping a god results in considerable unpleasantness. Of course some take other routes to avoid Urcaen entirely, such as the more extreme examples of necromancy. The religion of Lord Toruk is one way to attempt to avoid Urcaen, although I'm sure many of their living worshipers wind up dying in ways where their souls cannot be collected, and would likely experience all the same terrors of Urcaen as would afflict a non-believer.
The religion which is the most vague about their afterlife expectations is the Cult of Cyriss, but here again we see an emphasis on avoiding Urcaen by preserving the immortal soul in an ageless machine. This is a privileged status only granted to a small minority, but gives the faithful something to aspire toward.
On the other hand, Dhunians avoid the issue without necromancy by the process of reincarnation. Worshipers of Dhunia believe the goddess collects their souls into Caen on their death to be redistributed, which is another way to avoid the War of Souls or any concerns about the afterlife. Nonetheless, this is still one of the "reasons to worship a god" related to one's fate after death.
Death is a scary thing, and the fear of dying and what comes after is a prime motivator for just about every religion in the Iron Kingdoms, in one way or another.
As far as other pragmatic benefits, there are a number of boons given by many of their gods and their servants to their worshipers, even those who don't join the clergy. The benefits granted by various ascendants and scions to those who worship them, while small, are still appreciated by those who receive them. The pious of western Immoren certainly have a sense that the gods are real and tangible and may in fact actually look out for their interests (at least in some small and abstract fashion). For example, families that revere Asc. Gordenn are absolutely convinced that he aids the fertility, longevity, and general health of their family members. Those who are adherents of Asc. Sambert are convinced the works they craft are just a little better, more rugged, and less prone to wear and tear than if their patron was not looking out for them.