x1 is a spouse of x2's former spouse by law/custom/standard x3.
Given that Lojban is elliptical on tense, any past, present, or future spouse of x2's spouse could technically be referenced; usually, we will explicitly exclude the possibility of x1 = x2 except possibly via remarriage; additionally, if x2 were that person's nth spouse, then this word will typically (but not necessarily always) refer to a (n+m)th spouse of that same person, where n and m both are integer such that n > 1 and m > 1.