Hmmm... AU Tsukasa makes me see this convo in a new light.
Wataru: "Where are your buckle and cards?"
"Do you remember anything?"
Tsukasa: "No, just how to fight."
He wasn't asking out of concern for Tsukasa's well being, he was asking to make sure Tsukasa had no memory.
Our Riders know this is a puppet who they're leading around the worlds to kill the AUs before the real Tsukasa finds out what's going on.
And why would Natsumi discover the Decade buckle and cards randomly in some dirt pile in Ep 1? How would our Wataru know what they are if he didn't know Decade before? Because he was always aware there were two, and he was making sure AU would find real Tsukasa's equipment, send him across the worlds on a long journey, and real Tsukasa would be powerless to follow.
Until the Movie, where he catches up to him at last. Real Tsukasa has all the memories and knows the truth. AU was kept a blank slate in order for the mission to succeed, but he developed his own feelings and messed it up.
When Tsukasa meets Tsukasa, it would be the end for all the Riders, just like all of them coming together on War World has killed the AUs but kept the originals. They want him dead so that connection can't be made.