For an emergency or other situation like that I'd be ok with it. I think I would make them sleep in separate rooms though. Never a just for fun sleepover.
I admit, I'm not a sleepover fan. My "hangup" is that my DS comes home and is a complete nightmare to deal with. There is always some sort of argument, they never go to bed early enough, the get up too early in the am after going to bed late and I'm left to deal with the consequences. My DS never has enough "fun" for me to deal with his behavior the next day.
If he didn't constantly come home with a gripe and wasn't a complete bear, I might feel differently.