hey there,
if you don't care much either way, you could let them just make it up between themselves. you tell them both that you don't care, that you know/suspect the other supervisor wants something, and that they should talk to each other. then you're out of it.
if you DO care, that is, if you don't want the second supervisor's name on it, i guess you could discuss it with your first supervisor, letting him know that you agree with his position and that you would find it hard to talk about it with the other. basically ask him for help in this, and let him know that you can't deal with it alone due to the power imbalance between supervisor/student. so if he doesn't want the other's name on the paper, he has to do something.
just suggestions. good luck with that paper submission!