Imagins possesing Ryotaro have some to all level of control of him, but it costs them their mission, since he's a Special Point. Once they've possessed him, they're trapped. So, even if multiple Imagin possessed Ryotaro, even though it would probably kill him as well, the Imagin would be sacrificing their mission.
I have a theory that Ryuutaros was sent to Den-O to protect him, but the only way to get Ryu there was to promise him the train in return. This is why I've always thought that Older-Sakurai sent Ryu to Ryotaro. Imagin gain the properties of the person who imagins them, and his love for Airi means he either was spawned totally from Ryotaro, or from someone else close to Airi. The older rumors of the second rider said that Ryuutaros was originally possesing him. So, maybe Ryuutaros is actually created by Zeronos, and then sent back in time to give Den-O a fourth form. I can't see Ryotaro imagining up someone like Ryuutaros, even though they're slowly coming to an understanding.