These are old work boxes and are normally held in place with metal brackets that clamp the box to the drywall. These boxes were almost the perfect depth to screw directly to the framing so I pulled the metal brackets off and secured each box with a pair of #10 screws. This method will actually be a stronger than using the brackets as the framing will take the weight of the fixtures, not the drywall.

I did have to rotate each box 90 degrees as the brackets that came with the light fixtures didn't fit the wider set of mounting holes. That made the feed wire between the two boxes almost too short, but I was able to make it all work.