First off, I'm not a professional plumber, but have recently done my research on adding a sink in 2 different situations. The first set up I was able to tie in to the main stack for venting. In your pics I don't see any vent piping, hence the AAV? My second sink, was a utility tub away from main stack with AAV. It is crucial to place AAV under sink behind p trap and above exiting drain to work properly. The other consideration is if this plumbing location is lower than upper level drain lines. Check out Terry Love Plumbing online forums. He gives great advice for the DIY ers, without the snarky pro vs pro bashing.

Hi,

This is the basement level. I don’t know exactly where the current drain goes, as it is buried under the flooring. I assume it flows into the main stack...but I don’t see any secondary venting pipes.

The current sink #1 doesn’t have an AAV but drains well.

I was thinking that a single AAV above the 2 sanitary tees could satisfy the venting for both sinks...

if the current sink is draining fine, ( looks like what they call a wet vent) adding the AAV under new sink per instructions should work. Although the basement issue that may arrive is gurgling noises and possibly p trap may siphon out from upper level draining pushing toward new exit AAV if not high enough above sinks draining.