Issues running 2 legacy engines same time - help!

First, are you confident you have a good connection of the Command Base ground wire to the outer rail of all tracks?

Second, I have some beliefs on how TMCC/Legacy signals get into the house wiring, but till I get a chance to do some experimentation, I will keep these to myself.

That said, I humbly suggest you try the extension cord test. Unplug Command Base from its AC source. Using an extension cord that has all three wires in it (denoted by male end having the familiar two spade pins and on round pin), plug the extension cord into the wall, the Command Base into the extension cord, then route the extension cord along the track(s) you want to test. The extension cord now acts as a temporary antenna located next to the track free from any potential blockage from layout or wall elements.