Conventional and Legacy/TMCC at the same time

with command the voltage is a constant 18 volts, unless you are very fast the first time a conventional engine entered the 18 volt block it will go to full power.  you might be able to do it with two controllers. but if it was me no way. 

As long as there is a command signal, the command locomotives will be under remote control.  Because they are designed to run at full 18V, when they enter the lower voltage block that the conventional locomotive is on, they will slow down.  There are many people that use lower track voltage on command locomotives to limit their max speed as apposed to limiting via programming as they feel it gives finer low speed control.

 Yup, done it. I think the secret is to have a conventional engine that requires

 a fair amount of power. or cars that may require you to get to the 10 to 12 volt level to operate.

 Start with the conventional train, get the speed you like. then add the TMCC engine. If the layout is big enough you can put both on at the same time. If not

you may have to put the conventional engine into neutral.

It can be done....

Although this can be done, the results are about 99% unsatisfactory.  First off, there's the varying voltage. 

Another problem is for conventional you interrupt track power for direction changes.  When you do that, the TMCC/Legacy locomotive will stop, even after power is restored until it receives another "go" command.

Stoshu is correct that conventional engines and tmcc can be run at the same time.  His recommendation for getting the conventional engine going first is spot on.  The only thing that I see being a problem for you is the lock-ons.  They require a minimum of voltage to work and are meant to for constant voltage above 14 volts.  Gunrunner is correct about the direction changes stopping your TMCC/Legacy engines.  I run both conventional and Legacy at the same time at low voltages with a Legacy Powermaster and 180 watt bricks.

Thanks all. I thought there might have been some magic I wasn't understanding but while it does seem possible, the results aren't really satisfactory toward what I was looking for. Looks like I'll try an ac command upgrade or move out the old.

The ERR AC Commander will be a reasonable upgrade for your AC motored locomotive.  Of course, if you want sound, that's an extra cost.