Well, we don't leave them alone together. They will 'talk' to each other, Marty inside his cage, Romeo outside. When we let Marty out of his cage to play in his open play pen, Romeo will get in it with him and they will play. Romeo has even taken to laying down in the pen and Marty will cuddle next to him. Romeo has never made any move that would make us think he would hurt Marty BUT he is a cat afterall, and we are very, very careful and hovering over them. Romeo came to us de-clawed, but his teeth are still sharp!
And now that we have Tybalt, Romeo will lay on top of Marty's cage and swat at Tybalt if he gets too close. We think this is Romeos way of saying "Leave my buddy alone or else!"