This thread has details on the shim:
https://www.bikeforums.net/tandem-cy...i2-triple.html
I did the work on the bike early in the year, but we were out of town for a number of months. Finally back to some riding in the summer.
We very much like having the mid ring, makes cruising much smoother. Shifting the front with TA rings isn't perfect like a Di2 single
bike. This is expected. Still fine tuning, but it gets the job done.
The only downside is that I shift a lot more....
jeffs