Just as an FYI...
You only need to connect all TIU Commons together, with any other accessory or switch motor transformers, once. This is, of course, done after ensuring that all transformers are in phase with respect to each other.
The best place to connect the Common lines is on the TIU Common (black terminal) output. This is particularly true if one is also connecting the command "one wire" from a TMCC or Legacy command base to the commons of all TIU channels. If, instead, the Commons of the TIU's channel inputs are connected, the TMCC or Legacy signal is then passed through the TIU's DCS signal generators. While I cannot speak to exactly what kind of issues this could possibly cause, I simply cannot imagine any positive benefit that might be gained by dong so. 
