Most adult Gloranthans are an initiate of a cult, and a lay member of one or two others. So in Sartar, a typical woman would be an initiate of Ernalda (worshiping Orlanth Thunderous as an associated cult), and might be a lay member of some other useful cult or cults – let’s say she is a lay member of Maran Gor and Flamal.
Similarly, a typical man would be an initiate of Orlanth Thunderous, worshiping Ernalda as an associated cult. Perhaps he is also a lay member of Issaries and Lhankor Mhy (and is thus literate).
Sure as a lay member, all you usually “get” is access to spirit magic and training, but it often results in broader and nuanced characters.
Not many people are initiates of more than one cult, so few that it is almost a rounding error. A few exceptions exist – ALL initiates of the Red Goddess are initiates of two other cults: a Lunar cult (which they have become a Rune master of) and Nysalor (because they are illuminated).
Does the lay memberships have to be within the same pantheon or could you for example have initiates of Yelmalio as lay members of Zola Fel or Lhankhor Mhy? The rules for lay membership are much looser than initiates. An initiate can only become an initiate of an associated or friendly cult, but can be a lay member of whoever as long as they are not hostile or enemy cults.