Though it's technically possible to run simultaneously (if it's allowed in your state), it's only realistically done when you have a lock on your current seat. For instance, in 1988, Lloyd Bentsen was running for VP with Dukakis and also running to keep his Senate seat, but he'd been in it so long and had no real opposition that he easily won the seat even as he and Dukakis lost the national election, thus he was able to stay in government. It's usually easier than that for senators to run for other offices since they have a two in three shot of not having to run for re-election that cycle Bentsen just had the timing working against him. Representatives have to be re-elected every cycle which makes it a bit harder for them. Still, I'm sure others have done it since, though I can't think of any off-hand.
Crist is in a different situation. He just barely won his House seat and was likely to face a tough re-election match though he'd likely would have had an advantage. If he was busy running for Governor, it would easily be used against him by his House opponent. Basically, he'd need to focus on a single race or face the humiliation of losing two elections at once, which would likely end his political career.
