I've never used an air admittance valve for a double sink. I'll only use an air admittance valve as a last resort, like in an island sink where I can't do a loop vent. Other than that, sure, different fixtures can share drain horizontals and vents, they just need to be properly sized for the number of fixtures on the run and the length of the trap from the vent.

your thinking is about lowering the risk of cross-contamination to a certain threshold, deemed right. It is impossible to reduce the risk to Absolute Zero and any discussion of that leads to ludicrous scenarios, never likely, but still conceivable. The threshold is one that you will call "good" or that a kashrut inspector will call good, if he (or she) gets into that level of the building process. There are many ways to achieve what you seek, all leaving some possibility open that water sullied with compound A will come up the drain of compound B sink. If Even if a separate P trap were to be installed on all five drains (2 DW drains, 3 sink drains) it is conceivable (but almost never going to happen). Perhaps you'll consider a "proper for kashrut" preventative operation like closing the strainer on the appropriate sink, and cleaning it if you see the seal was not made. Since sink drains can be deemed to be dirty, perhaps you will decide to use grids on the sink bottom. Some sinks have grids suspended in space above the sink bottom (they rest on ledges on the side walls, e.g. Franke sinks like the Orca, and e.g.2. you can solder a ledge on a stainless steel sink), and this may be deemed one step closer to fine. If your sole concern is about dirty water when it is hot warm or lukewarm, then lengthening the drain can resolve that single concern. How you do it will depend on your licensed and insured Master Plumber and his professional decisions given your space and your DWV constraints (your plumbing's venting, your DFU's, and your pipe diameters downstream of the five drains). Only he (or she) knows for sure. Not a repair plumber who may often be called upon to work on P traps and drains. HTH -david

This article is fairly old, but might give you a start on figuring out the plumbing. http://terrylove.com/forums/index.php?threads/back-to-back-sink-dwv-fitting.17788/ However, the exact layout also depends on what is already in the wall (vent, etc,) and the size of the drain on the existing sink. Bruce

I don't have the sink/counter installed yet but I'll post a picture of what I got going on. Also have 2 drain pipes so I'm thinking if the one is to high I could use the other one.