For actual separation I’d start super small like each getting an amazing treat in different rooms before immediately being reunited. If that’s too much start with separate ends of the same room or separate sides of a baby gate. Also there’s no shame in meds, anti anxiety meds can change everything!

Working on turn taking can be helpful with multiple dogs. A place cue with beds set up so one dog is resting and one is working. This can get them started on the idea of working separately while still in the same room.

Snowballs make the best fetch if you ask them