Michael, it is a bit unclear what you mean by "to build your own Multi [Simultaneous=...]".
For different or for unique project?


A)Your own recording via user created User Track - This will work for different projects well enough if you stay within reasonable tempo change from original.

B)Opening another instance of (Trumpet / Trombone) RealTrack on another mixer track. The "simultaneous" part is a bit of artistic choice. There is a tool called Partial Regeneration. You can use that for a better fit. However it will sound different in another project as RTs are phrase based. You will have to use Partial Regeneration again on a new project with different chord progression.

C) There are also "playable" Real Tracks. I don't have experience with them, but someone here will likely help you.
To my understanding, if they (playable real tracks) have share same MIDI chart, they should in theory sound "simultaneous".