Hi - I do not have the same midi keyboard as you, but tried to replicate your test using a Yamaha keyboard.
I inserted 2 midi tracks and recorded two different tracks of midi data, with the output of each track going back to the Yamaha keyboard.
I did not experience the same issues as you did when I tried soloing each track - did each one in different order, then had both selected.
In all cases, sound played back as expected:
1. Neither track soloed played back both sets of midi data
2. Track 1 soloed played back just track 1's data
3. Track 2 soloed played back just track 2's data.
4. Soloing both tracks played back both track 1 and track 2's data - just like having neither track soloed did
In the above, I did NOT have instrument tracks - I have never used those - I just inserted 2 midi tracks.
Please forgive my ignorance here - I understand the TI to be a physical keyboard - how would I create a Simple Instrument track for this device? It is my understanding that a Simple Instrument track can be created through inserting a soft synth, as a result of having the box checked in the insert soft synth options dialog box, and that a simple instrument track is a merged track with both audio and midi being stored in the same track.
If my understanding is correct, then how would I be able to create a simple instrument track for my Yamaha keyboard? If you can explain this to me, I would be happy to try and test this all out using my Yamaha keyboard.
Bob Bone