They appear to have run out of slope and entered the main side. There wasn't much you could do. To solve a problem that hasn't materialised, you may lay whole new pipe.
Both have a drain that exits through the base of the cabinet and an AAV above the Santee for the p-trap arm. I had the same problem with two sinks connected in series and one p-trap vented through an AAV, and I engaged plumbing services to fix it.
The best course of action would be to link them all in the crawl area after running each drain to the outer wall and bringing them down the wall separately.