there's a trick where, if you plane two edges at once, they'll fit together even if they aren't perfectly square to the face
How does one go about doing that?
I'm going to take a stab here and guess that the intention is that if you take two boards clamped together you could plane two edges at the same time (assuming your plane blade is wide enough). Those two planed edges would fit perfectly together if you rotate one board 180 degrees before fitting them together. This is true even if you're not planing at exactly 90 degrees or square to the face.
Assume for a moment that you plane at -- say -- 75 degrees to the face. Now you have two 75 degree angles. If you try to just join those two edges you'll obviously not have a flat board -- you'll have a 30 degree angle (or 150 depending on how you want to look at it). If you flip one board 180 degrees (e.g. take the board on the right of your 2 board sandwich you just planed and rotate it such that its outer face is now the inner face of the sandwich) then you've effectively turned one 75 degree angle into a 15 degree angle such that when the two edges touch you have a perfect 90 degrees.