[kamocat]

Here's a way to get around the issue of putting two motors on the same transmission:
Use only Tan Jaguars.

The reason is, Tan Jaguars switch between 12v and open (coast) in their switching.
Black Jaguars switch between 12v and 0v (brake).

Thus, if one motor is driving the other faster, they won't conflict; the slower motor will simply coast along.

This means an encoder can be split off and wired to two Tan Jaguars for closed-loop control without issue.