Dave,
From what I can see I don't believe you will have any issues. I have the Legacy Base wire going to the Variable 1 Out (-) post on the TIU. (Since it is a common (-) it doesn't matter which Line Out post it is connected to.) Then I have the appropriate Line Out (+ & -) going to the A & B terminals of the MTH PCBA. Then run both + & -, wires to each "block" of the desired track. (Coming from the terminals of the PCBA.
On my "table" layout I have done the same thing for 4 different areas. I have three sometimes four seperate tracks in one area. If in that area I need 2 track connection points for 1 track, and 1 track connection point for the other three, then I cut the etch on the back of the PCBA, and essentially create four pcba's. Example: Track 1 needs two connections. Will have the input wires to the PCBA coming from Variable 1 of the TIU. Then connect two sets of wires coming from the PCBA to the two sections of Track 1. Then connect a set of wires coming from Variable 2 going to "say" terminals 4a/b, then run a set of wires from PCBA terminal 5 to Track 2. I cut the etch on the back side of the board between terminals 3 and 4. And cut it again between terminals 6 & 7. I used terminal 6a/b to run a set of wires to the NEXT area and the distribution PCBA for THAT area. And continued the process.
Hope this makes sense if not send me an email at dbaker48@live.com.