Upgrading from a single-basin sink to a double-basin sink is a great choice to increase the amount of space for washing dishes and preparing meals for adding to the getting-ready space in a bathroom, but the plumbing configuration is slightly different for this type of sink. Instead of having the tailpiece from the single sink connect directly to the P-trap drain, each sink in a double-basin setup has a tailpiece that connects to a sanitary tee, which in turn connects to the P-trap drain. (Though you can also use a combined swing tee trap.) This configuration directs the flow of waste from both sinks into a single drainage line.
Beyond this major difference in the plumbing configuration, single-basin and double-basin sinks follow the same basic installation process. Even the hot and cold water pipes remain the same, because a double sink still uses just one faucet. Intermediate-level DIYers should feel comfortable tackling this installation process with the straightforward steps below.
Before You Begin
You cannot install a double sink unless you have the necessary space, so make sure to measure the area, compare it to the double sink that you want to install, and cut holes that allow all sides of the double sink to sit on the counter. The sink basins should fit through the holes, while the edges sit on the sides of the counter to support the weight of the sink. The size of the holes remains the same, even if you are installing an undermount sink (though undermount installation will be much more challenging).
It’s also important to turn off the water using the hot and cold isolation valves under the sink so that the faucet can be installed. If the current plumbing does not have isolation valves, then you will need to turn off the water at the main shut-off valve and drain the water lines in the home before starting.
Source: The Spruce
