I have a leaking sink drain (below the trap). I've cut out the back of the cabinet to gain access and there are 2 tee's which are stacked directly on each other. How do I cut out the lower (leaking) tee without interrupting the upper? There's limited room to move and once I cut the lower tee out, how to I reconnect to all the existing pipes? I can send photo if that's helpful.

I have a leaking sink drain (below the trap)
That means you have a leak in either the trap arm or in what plumbers sometimes call the dirt arm (the horizontal pipe that comes out of the wall). These can be repaired or replaced.
Show picture. Upload pic to computer in JPEG format. Make a post in Quick Answer block. Click on "Go Advanced", click on "Manage Attachments", Browse and find picture, upload to this site and attach to your post.

Where is it leaking from ? Picture would definitely help. There is a chance, you can connect both sinks into same "T" and abandon the other one. Please, post a pic.

Milo