Question: if Jyn Erso and Captain Rex are on the same ship, does a single Squad Command or token trigger them both? Or would they need a Squad token and a command in order to go off at the same time?

Jyn requires that a squadron token or dial is spent for her effect.

This does not resolve a squadron command that is needed to trigger Rexs ability.

You could, for example, have a squadron token and reveal a squadron dial.

Spend the squadron token for Jyns effect, and then resolve the squadron dial for Rexs.

CloneWars bros Rex/Kanan is where 3 raids from one token is at...