UPDATE: Sorry, I see you already have a plan, which sounds like a good one. Using the Legacy compatible cable should allow you to disconnect the U wire from your TMCC command base from the track, but continue to use both the old and new CAB remote, as well as the MTH remote.
Mike,
Using the original TMCC Base and the Legacy Base connected via the Lionel serial Y cable is fine; that allows you to use original TMCC CAB1 remotes and Legacy CAB2 remotes simultaneously. As you indicate, that worked on your layout. In this configuration, there would be a ground wire from the Legacy U terminal to the track. On the TMCC command base, there would be NO such connection.
If you connect the U terminal on both the TMCC base and the Legacy base to track on your layout, even if these tracks appear to be isolated, you will have problems.
