Possible to run one TMCC engine via remote, one conventional?

I agree with Mark, nope. If I want to put my locos into conventional on the workbench, I have to shut down the layout. The nearest source of track signal is three feet away from the bench. Once an engine is exposed to the signal, it locks out conventional (if it's working properly).

I did run my Post War 681 simultaneously with a TMCC Allegheny thinking it was very nice to have two engines running on the same track with more than 50 years separating their build dates.  To do this I loaded the 681 with a bunch of cars so that at full power it ran normally and did not bolt down the track.  Keeping the voltage between 15 and 18 volts, you can run the TMCC train and modify its speed to sync its speed with the conventional engine.  I ran my post war this way until I had enough command engines that my post war engines got relegated to my son at his house.

TMCC is a radio signal that uses the track's outside rail as a handy antenna.  The signal is broadcast through the air so any TMCC loco in the vicinity will "see" that signal, even if it is on a different track several feet away.

You may run TMCC and Command locos on the same track at the same time. The process is called "transitional command control"  Basically, you will need a command base (Legacy, TMCC or Legacy 1L) and a Powermaster ( Legacy or TMCC).  I refer you to The Complete Guide to Command Control, Page 30, Lionel Part # 71-2811-250.  It was published and available in the early days of TMCC and was included in the Base, CAB1, Powermaster package sold some years ago.  Although it applies to TMCC, the procedures outlined are just as apropos to Legacy.  It takes some skill, but is sure is fun to run a Command controlled  train and a conventional one on the same track simultaneously.  You won't need to retire your conventional fleet or run them separately, if you so choose.   Check out the manual.  It is extensive and makes for some very interesting reading.

the easier way to run both command and conventional would be to get a few post war engines and let your boys run them. the post war items could care less if a lionel/mth command base is around and you can enjoy the bells and whistles on your track.

Tried this myself years ago until I learned that connecting the Command Base to one track's outer rail simply turns the track into an antenna and any TMCC engine will pick up the signal regardless of which mainline it's on.

I think STPAUL has the best idea....just get a conventional engine or two for use with the transformer. Heck, with the price of postwar engines cratering and modern conventional not fairing well either, it should be easy to pick up gently used conventional for $100.  I just sold a Like New conventional WBB Hudson for $150 and was elated to get that price.

If you're not wedded to scale, LionChief or LionChief Plus engines will give the boys remote control. That's a pricier alternative, I admit.

For my Christmas layout this year I decided to run it conventionally. To keep my Legacy system from interfering with that I disconnected the antenna inside the engine, thus making it conventional only. I'm did this with a steam engine, doing this with a diesel may not work it's it got a plastic shell.

Yes. The trace on the Radio card is usually long enough to be an antenna 

I test stuff on the bench all the time with no antenna, Chuck is right.  If you want it to work, try taking the antenna connection from the motherboard and connecting it to frame ground.

Thank you all for the advice.