I doubt if a divorce would be necessary. The JW's would most likely take the stance that same-sex marriage is no marriage and is therefore null and void right from the start, although this could cause legal problems, especially if the abandoned one found someone else. I would imagine the newly-converted one would have to cease dwelling with his partner and would most likely even have to give an undertaking never to be with him in an unchaperoned situation.
Quite why JW's think the good Lord has nothing better to do than to hang around watching what we do with our genitals is quite beyond me.