Great question — answering as the UphillTech team:
That setup looks okay at first glance but it's the kind of thing that bites you. When one sink runs and the other backs up, the water is meeting resistance downstream and taking the easy path up the other trap instead. First thing I'd check is slope on that shared run. Even a flat stretch will let water sit in the pipe and slow both sinks down, and from your picture it looks pretty level across there. Both arms should fall steadily toward the tee. The other usual culprit with double vanities is how the two arms join. If they're just tee'd straight across from each other with nothing between them, the discharge from one side shoots across and pushes right up into the other trap. A double...
Full answer by Aliha Rao: https://diy.stackexchange.com/a/332310 (CC BY-SA 4.0).
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