I put a quick rundown in the linked post (Rummikub on the Glowforge, a Journey – Shutdown Hook); this is the best guide I’ve found: Glowforge Double Sided Engraving Procedure …
Basically you create a design that has all of the elements in one upload, fix the material in place, then do two print runs. First print does the cuts and front, setting the back to “ignore”. Then flip each cut piece over and put it back in its hole and print the back, with the cuts and front on “ignore” this time. Keeping it all in one design ensures the alignment works — super cool! Only works for pieces that can be cut with a symmetrical north-south axis, but that’s a lot of good stuff.