Good question, Jen. I have a special board that allows me to set a 5 frame nuc on top of a 10 frame. It's looks similar to an inner cover, but there is an opening that fits a 5 frame nuc. The tops of the other 5 frames in a 10 frame hive body are covered. Before I got that, I would have moved the nuc into a 10 frame and did a newspaper combine. I would put 5 extra frames in with the nuc frames so that there isn't empty space for the bees to try to fill. I would normally put the weaker colony on top being sure to feed the top colony with 1:1 sugar syrup.
If the nuc is queenless, there shouldn't be any problems combining the two. The logistics are awkward. So, that being said, I would move frames to boxes that are the same and fit each other. Put the weaker colony on top with a sheet of newspaper in between. Cut 3 slits in the newspaper and use lots of smoke. Feed the top and weaker colony.