Allow me to add a little more to what has been said.
Once you put the two dough pieces together, resist, with all your might, the temptation to pull the larger dough pieces around the smaller one. Just press them together as suggested, then press it down sufficiently to get it started through the sheeting rolls if you are using a sheeter, or if you are hand forming, I lioke to press the dough down on the bench top to flatten it a bit as part of opening the dough up into a skin. Additionally, try to keep the smaller dough piece centered on the larger dough piece when combining them as this makes it easier to open the dough ball without problems created by the two different dough pieces.
Tip: If you diccover that you are out of a certain size dough ball early enough, you can resize the dough balls, by putting the smaller, add on, dough piece on the bottom of the larger dough piece, then place the dough ball back into the box until needed. This wil allow more time for the two dough pieces to knit together, and it also allows the new weight dough ball to relax a little more too, allowing for easier opening of the dough. To make a dough ball smaller, yust lop off a portion of the dough ball until you get the weight you want. I like to go around the edge of the dough ball cutting from the different sides trying to maintain something that still looks like a round ball, if you just lop off a chunk from one side of the dough ball you will end up with a mis-shapen dough ball that may be a bit more problematic to open into a nice round dough skin. If you want, you can save all of those trimmed pieces and incorporate them into your next fresh dough batch (keep them in the cooler), or, you might even be able to carefully combine them together, making dough balls that you put back into the cooler or allow to ferment at room temperature, depending upon how fast you might need them. By carefully, I mean do not over work the dough when forming the dough ball, just gently press the dough pieces together, given sufficient time (about an hour or so) they will knit together allowing you to open it into a dough skin without it coming apart at the seams.
Tom Lehmann/The Dough Doctor