I've done it a couple times before. What normally happens if one has unstoppable and the other doesn't (me being the one who does cause I'm cool like that) is that the one without unstoppable dies, but it all can change so much depending on the exact moment that both Titans activated their supers, how far they are from each other, wether or not they have aftermath, all of these things can completely change the situation. But really though, most of the time both live and it comes down to a straight up gunfight like every other non-super engagement(this is where aftermath comes in)