Allan,
I'm just not sure how/where to connect the Legacy Base to that same track loop.
The following is from page 122 of The DCS O Gauge Companion 2nd Edition:
Wiring for TMCC on a DCS Layout
Regardless of the method of control chosen, either Lionel command base is wired by connecting it's "one wire" to the tracks as instructed in its documentation. Alternatively, the Command Base's "one wire" can be connected to the TIU channel output Common (black) terminal of each channel on which TMCC engines will operate. If it is connected in this way, it is important to note that only the original, Rev. G TIU has the Common terminals of all channels connected internally and later revisions of the TIU do not. If a TIU other than a Rev. G is used, all Common (black) terminals of TIU channel outputs should be connected together, and to the Command Base, to ensure that the TMCC signal is present on all channels. Alternately, the "one wire" can be connected to any one powered TIU channel as long as all the transformers that are providing power to TIU
channels are in phase and have one of their Common terminals connected to the Common terminals of all of the other track power transformers.
Regardless, if both the TMCC and Legacy Command Bases are both to be connected to a TIU, only the "one wire" from the Legacy Command Base should be connected as described above.
If control of TMCC engines from the DCS Remote is desired, the MTH cable is connected between the Serial Port on the TIU and the Serial port on a Lionel command base. If both the TMCC and Legacy Command Bases are to be connected to the layout, the TIU is connected to the Legacy Command Base. Regardless, only one command base is connected to one DCS TIU.
There are a couple of wiring diagrams, as well.
This and a whole lot more is all in "The DCS O Gauge Companion 2nd Edition", now available for purchase as an eBook or a printed book from MTH's web store site! Click on the link below to go to MTH's web page for the book!