How do I recall Gooigi in co-op mode?

Solution 1:

If the player controlling Gooigi clicks down the joystick twice it recalls Gooigi to Luigi.

Solution 2:

You could Exit the room.

Sure it's not the button combo you are looking for, but while Gooigi is off doing his business you should genuinely have enough time to do your part as Luigi in the room you entered.

If Multiplayer mechanics are anything like the single-player ones, Gooigi should return to Luigi if you leave the room, so just let Luigi clear the room and have a little patience. From what I have seen so far, there are not that many passages where Gooigi is separated from Luigi and can't return anyway.

(Well now I remember the secret room in the shopping mall, but that might be an exception.)

This is not the final answer, but it's a temporary help I guess.