My answer depends only on what marriage is, and what it is not.

I would not even consider any "marriage" after the first one to be a real marriage, i.e. in the eyes of God, in any culture no matter what the participants agreed to, just as I would not consider two men's "marriage" to be a real marriage.

Since adulterous relationships are not marriages, the breaking of the relationship is not divorce, in spite of significant emotional attachment and sharing of material and intangible goods and life by the participants.

This, of course, would be extraordinarily difficult for the former Muslims but not more difficult, I would imagine, than the situation of two men with homosexual inclinations previously married in Boston but now converted to the faith.