Yeah, just did a test vs the camera crew. Mirror Move simply copies the last move that targetted the user, regardless of the source, using the normal 1-on-1 rules (certain moves can't be copied for example). Relative speed plays a big part in all this - your opponent can derail using it to copy a partner's attack if they act *after* the partner. Most reliable if the Mirror Mover acts first and the partner acts last naturally, with the slow one attacking its buddy at the end of one turn and the buddy using Mirror Move at the start of the next turn.
Different moves have different effects in 2v2. There are a class of attacks (iei Earthquake, Dizzy Dance) that target everyone but the user, some that attack both enemy targets (i.e. Surf, Hyper Voice) and some that seem to choose an enemy at random (Outrage for example). Most just attack one selected target as normal. Light Screen and Reflect protect both partners.
Moves that affect more than one target usually have a reduced power vs each target - note that non-damage moves that affect more than one target like Tail Whip or Dizzy Dance become much more powerful in 2v2.