What would two siamese twin sisters fight about?

I'm writing a story, and two of the characters are sisters who are siamese twins (or rather, conjoined twins, since that country is now Thailand). They basically have two heads on a single body. There's one scene where some other characters walk in on the two of them having a spat, but I'm not sure what a good subject would be for them to have an argument/fight over. Any ideas?