I've been playing the Thunderspire Labyrinth adventure for D&D 4th edition, and at one point a character was grabbed by two different monsters.
Can two different creatures of different types grab and sustain a grab against the same creature?
Yes. It would be same two people holding/restraining you as long as both creatures want to cooperate. If they are into cooperating then the character is going to have a tougher job breaking the grab of both.
If they are not into cooperating then if both grabs succeed there may be a check for the first 'grabber' resisting having it's 'grabee' stolen away.
Can you say bungee cord character?
If you're grabbed by multiple attackers, to escape you make an ordinary escape check, with +2 to the DC for every attacker beyond the first. I don't have a page ref right on hand, but this happened with my group last weekend -- they were attacked by stirges (which were nasty!).