Double-vanity sinks drain slowly -- venting problem?
Going from a single sink to a double should not cause any problems. With the vent being close less than 6' from the drain that should also be ok. When I plumb double sinks I only use 1 trap, you mentioned 2 traps so the sink that is further offset from the drain may not drain as well. I would reduce down to 1 trap just prior to the wall and see if this solves your problem.