What can I do to rescue overwhipped cream? It just separated in my rather warm kitchen. I just want to have tiramisu tomorrow!

You can try to incorporate a little more cream - add some to the side of the bowl and gradually whip it together. If it's well past over whipped/separated then this won't work.

Did you over whip it? Usually if it separates you are on your way to making butter. Try whipping in a bowl that is sitting in another bowl of ice to keep it cold. I once whipped cream while it sat in a bowl of ice and frosted the cake by sitting it in my freezer, it was so hot in my kitchen it turned instantly to soup. It worked because it kept the whipped cream cold while whipping and frosting the cake.