Yes, you can nominate as many as you like.
I am not aware of any state that limits the number. But it is not the best idea in the world. Not only can the logistics of co-executors be a problem, but there is so much potential for conflict and stalemate.
Remember that the executor is supervised by the court. The executor does not have free rein to do as he/she wishes and the court is there to redress any wrongdoing. While it can be considered a compliment/honor to be nominated as the executor, it can also be a real pain-in-the-rear duty.