I assume you are fearing/sheeping the targets and focusing on main.

If it's a one-shot deal, you can probably do something like this:
/cast [target=warlock-target] Sheep

However, if you are looking for a real switch of targets, I think you have to do it in two steps (two macros). First have the warlock and mage focus each others target. Then, they can target their focus and focus on main again.

The problem is that they need to keep their target until the other alt has registered it. If you try to do it all in one macro, you end up with a race, and they are likely going to end up both with the same target.