All of your Legacy and/or TMCC devices need to have the same daisy chain (dat/comm) from the Legacy Base. This is so that all these devices: ASC's, BPC's, TPC's, etc. can receive their commands from the Legacy Remote to the Command Base and then on to the devices.
The "U" connection is to provide the Command Control Locomotives the commands from the Legacy Remote again through the Command Base to the "U" connection to the outside rail (Neutral) of the track and then to the Locomotives.
Having said the latter, the "U" does not in essence need to go to the TPC but directly to the track. The Neutrals are conceptually all the same and the "U" could go to the U of the "Track Out" if this is advantageous wiring wise to get the Locomotive Commands to the outside Rail.
All my Neutrals are tied together aka "Common Ground". So you can insert the "U" to any central wiring block to get the Command Control Signal to the outside rail. So to answer another question I think your asking, the upper and lower levels can have their "Neutrals" tied together and Hence the Upper and Lower will receive the same Locomotive Commands from the Control Base.