I'm installing a single sink with a disposal in my kitchen. The previous setup used a double sink and disposal. The drain is plumbed for a double and as you can see from the pic, the drains go left and right instead of straight out from the wall. There's a coupling between the drain "flange" coming from the wall and the other flange with the 8 sides to it. Everything is glued. Even the 8 sided piece is glued. What can I do?

I would cut the PVC through the straight collar - about 1/4" from the back (wall side) of that straight collar. Once you cut everything off, then carefully cut the remaining parts of the collar off the 1-1/2" PVC beneath it. This will give you a 1/2" stub of 1-1/2", more than enough to attach a new collar. From there, make up your trap as needed. Keep everything schedule 40 PVC and only convert it back to light gauge at the sink tailpipe.

If you don't want to do that, then cap off the threaded side of the 'Y' and plumb through the old trap.

Thanks for your reply Jaybee but I didn't understand what you meant by cutting the straight collar. I didn't see anywhere I could have cut and still have enough pipe to attach a collar. I ended up going to HD and asked the "plumbing guy" to suggest everything I might need to finish the job. Of course he didn't think I'd need a 2" coupling - so now I get to back to HD!

what you need is a fitting drill bit
they make a drill bit, that will remove the pvc from inside the "hub" of the pipe you have against the wall. this will allow you to then build on to the pipe with your new drain configureation
cut the pipe even with the "hub" that is sticking out the wall, buy the drill bit. [ i'll let you borrow mine, u pay shipping back an forth lol]
get it at a plumbing supply house, you need a 2in pvc fitting drill bit