What is the best way to group marines and medivacs?

In one group or two separate groups or both(one with marines , one with medivacs and one with both)?

Whatever you’re comfortable with. I personally put both in one group

I just make my current out group of medivacss number 2 and manual the mission. never had a problem.

I switch as I go. Medis + marines in 1 hotkey, then i drop, ctrl select the marines, assign them to the same hotkey so they can move freely under stationary medis.