I would say that, in those terms, the equivalent of the divine magic you once gained from associated cults is learning a stand-alone feat from another cult.
We know that feats are learned on Heroquests. You follow the story, do the action as the god did, for the first time, and gain the insight , as they did, that enables you to do that magic now in the middle world.
At some points your myths cross those of others. So your Issaries myths cross with Orlanth's myths. Where that cross-over is a point where the other religion goes to learn a feat, you can probably learn that feat too, by doing your quest up to that point, then taking a detour to learn their feat.
I suspect that you can eliminate the dangers of this being experimental heroquesting because your friendship with that cult means that they will pass on to you the secret of what to do there to learn that.
I suspect these points where your myth crosses theirs at the point that they learnt something new are few enough that there are a limited range of opportunities to do this. But YGMV. in that case the RQ associated cults are just abstractions of the commonly known points where this kind of exchange is possible for those cults.
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