To plumb a double sink, install two drainpipe valves to each basket strainer and connect the drainpipes to the tailpiece. Use a t-fitting to connect the drainpipes from each sink.
To replace a single sink with a double sink:
- Turn off the water
- Remove the vanity
- Turn the water back on
- Install the drain
- Attach the tailpiece
- Install the vanity top
- Connect the valves
Shut off the water both at the shut-off valve under the sink and at the main shut-off valve to the house, and then drain the pipes by turning on the house's lowest faucet.
Remove the existing sink or vanity by cutting away the existing caulking. Take care not to damage the wall, counter top or vanity base. After the pipes are drained, remove the existing shut-off valves and replace with double-outlet valves.
Turn the water back on at the main supply valve, and ensure the new valves are not leaking.
Apply plumber's putty on the drain trim and push down the drain assembly into the putty. Scrape away excess putty to create the seal. Attach the flexible supply lines by tightening the compression nuts onto the faucet's threaded posts.
Screw the tailpiece onto the drainpipe, and then attach the lift lever and stopper.
Attach the new vanity to the wall studs, ensuring the valves have enough clearance. Use silicone around the vanity and place the sink on top of the silicone.
Connect the hot and cold supply lines to the shut-off valves by tightening the compression nuts. Use a t-connector to connect the drain assembly to the drain pipe. Finish the job by caulking around the vanity's top and sides.