I've seen it go well, I've seen it go miserably. Overall, there's been enough miserable that I have grown wary of OOC couples who RP on the same guy.
@SG mentioned (oh so eloquently) one partner telling the other what's what, OOC. I've seen that happen more than once, albeit not in such an overt situation. I've seen @GangOfDolls' situation, as well. And like @Coin just said, it's not limited to OOC couples, but I do think I've seen it happen a little more overtly when it is OOC couples.
So, in all, I guess I'm slightly skeptical about OOC couples, and definitely more skeptical when their IC opinions come to be suddenly aligned.
I also can't stand the OOC angst, anger, snide comments, etc, that I've seen an OCC/RL partner pull, when the other partner's IC persona is flirted with!