When we stay with my in-laws we sleep in a king sized bed made by pulling two twins together. It works, but it isn't ideal. We're happy sleeping on it for the 2-3 weeks a year we are there, but I don't think I'd want it for our regular bed. Sorry, I know that's not what you want to hear!
If you use two twins, like the pp said, you will have a ridge in the middle. It might be ok, comfort-wise, but I don't think you'd be able to get rid of the ridge completely. In the end, DH and DD1 slept on one side of the ridge while DD2 and I slept on the other. DH and I like to snuggle in the middle with the girls on the outside edges of the bed (of course with bed rails!) But that ridge can really get in the way!!
If the ridge wasn't there, I can't see any other reason not to go with two twins. We use regular king sized sheets on ours.
There might be ways to get rid of the ridge - a feather bed? one of those egg carton shaped pads? I don't know how you'd test that without shelling out all that cash, but maybe they'd work?