I just tested this the other day with two Red dimmers.
First, pick one of the dimmers; if you have smart bulbs make it the one that isn’t associated to the smart bulbs. Set Parameter 12 to be 11 (per EricM). This may take a tweak to make Parameter 12 appear as an option. This part is essential though as you get a loop without it where the switches keep trying to update each other until you pull the airgap switch.
Then associate both dimmers to each other with Group 3 and Group 4.
And then it works. Two dimmers, with LED bars that stay in sync.