@BillYo414 and @DLC I think you all are talking about the Legacy Powermaster controller which comes in two wattage varieties. Legacy Locomotives and Conventional can be operated at the same time, if you make the necessary track power isolation.
You could, but wouldn't have to dedicate one or more specific tracks to running just conventional and others to Legacy. If you wire your track with electrically isolated power districts on each loop and yard, siding, etc. you would have the ability, with a track power switching array, to control which track district(s) the variable output of the Powermaster (for conventional control) would go to. This is also assuming you want to be able to run conventional locos with the Legacy remote. There would also be the option to just use a variable transformer to control the conventional locos without the Powermaster, and still use the Legacy remote to control Legacy locos.
In addition to deciding which track(s) get variable voltage (either from the conventional transformer or Powermaster), the same switching array could also provide 18V to the other districts for running TMCC/Legacy, (and if you want) LionChief, all at the same time. The switching array might sound complicated, but in practice it's pretty easy using something like the Atlas #215 track selector. Each #215 selector has two power input sources (A and B) with 4 selector switches, one for each of the track output terminals. Each of the 4 switches can select either A, off or B. The #215's can be daisy chained to provide the same two power sources independently to more than 4 track power districts and/or blocks.