If you installing a double bowl sink, the two bowls must be plumbed into a single drain. For most of the house, the drain lines are on the rear wall between the two sinks. So, connecting the drains requires double tee fittings. Water is routed through each drain into this single fitting, which flows into your house drain system.
The best method is to use PVC drain kit designed for double sinks. PVC is user friendly and the kits require no glue.
1.Place a slip nut over the short curved end of one of the 90-degree elbow pipes. The threads on the slip nut should point away from the pipe. Place the short end of the pipe on the bottom of the left drain basket with the long portion pointing toward the center of the two sink bowls. Thread the slip nut onto the drain basket just until it is hand tight.
Repeat the process on the second sink bowl. The ends of both elbow pipes should be pointing toward each other. Place the double tee fitting against the ends of the pipe. Make a mark on the pipes where they need to be cut so they each fit snugly into the tee.
2.Cut the end of each elbow pipe with a hacksaw. Slide slip nuts over each end with the threads pointing toward the ends of the pipe. Fit each end into its respective port on the side of the tee fitting. The tee should be pointing down. Loosely thread the slip nuts over the tee fitting.
3.Insert the wall pipe elbow into the house drain line coming from the wall. Slide a slip nut over the pipe with the threads pointing toward the wall. Loosely thread the slip nut over the end of the house drain line.
4. Slide another slip nut over the end of the wall drain pipe. Butt the sink P-trap against the end of the wall pipe and thread the nut loosely onto the sink trap.
5.Measure from the flanged end of the T fitting to the flange on the top of the P-trap with a tape measure. Cut a piece of straight pipe to the measurement with a hacksaw. Place a slip nut over each end with the threads pointing toward their respective end.
6.Push the straight pipe into the bottom of the tee fitting and into the top of the P-trap. Thread the slip nuts over the P-trap and tee-fitting threads. Tighten all slip nuts until they are hand tight. Turn each nut another half turn with a pair of slip joint pliers.