TMCC/Legacy Question with Double Road Numbers

Yes

They both must have a unique ID.  The only issue I see is if selecting by roadnumber.  But there are work arounds for that too.

lots of people have engines with the same road number.  just get some paper and write down the engine number and pick another number to put into the cab2.

Cool, thanks for clarifying that for me. That Legacy 990 will be my next purchase. 

I use the last TWO numbers of the Engine Number in my Legacy Base for each Engine. If I have two numbers alike, I use the FIRST two numbers of the Engine for the second one. You can always use the "Scroll" function on the CAB2 to find it!!!

FREDSTRAINS

I re-read your original post.  I think I might have gave you a correct answer but not the one you are looking for.

When you load the module for the new 6-38739 it will obviously assign the road number matching the older engine.

You can select the older engine with the new shell and I assume a different road number and edit it's road number by pushing INFO.  Once there you can scroll through the options and change the displayed road number to match the replacement shell.

Now both engines even though technically are the same, but with different shells will have their unique road numbers as well.