Sharing Units

It's very simple to have shared unit control between two or more players. Create your trigger, with whatever events and conditions you desire. Then, add the following action:

Player - Make player 1 and player 2 treat each other as Ally With Shared Vision And Control

This will allow players 1 and 2 control each others' units.

You can turn off control of your own units, so that, say, Player 1 can't control his own units.

Limitations

You can't share unit control of a single unit; only all units may be shared, or none. A workaround is to have a third player, and give the single (or more) unit to that player. Then share control of that player; however, this requires having a spare player used for essentially nothing else.