As Kage says, you can visit one Aman while staying at another. However, you are talking two days here, and the trip between Amankila and Amanusa often takes two hours each way, so I would never suggest doing this. Also each and one Aman at Bali is a wonderful destination in itself, also Amanusa, so no need to leave this resort to go somewhere else anyway (unless you have a lot of time or you REALLY want to see both).