Make mercs share items like the private stash.
Just remove the items and equip them on your other merc.
That’s annoying and confusing, I can’t farm items for the templar playing as a wizard or farm items for the enchantress playing as a Crusader.
It can be annoying, but you have a couple options here.
You can just equip the off stat items and live with it or you can reforge their gear to the main stat that you want them to have.
You can reroll the main stat on an item dropped by your level 70 hero at the Mystic with a level 1 hero of str, dex or int class.