If you are under the UPC (Uniform Plumbing Code) you may not combine two fixtures on the same waste arm.

If you are under the IRC (International Residential Code) one center trap may serve up to 3 sinks, lavatories or laundry tubs of the same type, in the same room providing the maximum horizontal separation between any two fixtures measured from the center of the drain openings does not exceed 30" however the trap and fixture arm diameter MUST BE appropriately sized for the combined DFU (drainage fixture unit) load.

A lavatory sink is rated at 1DFU therefore your proposed combined DFU load would be 2DFU’s.

The code standard drain opening size for a bathroom lavatory drain is 1-1/4" and the trap and fixture arm are supposed to be sized the same as the fixture tailpiece, which would also be 1-1/4" and rated for a maximum of 1DFU however, the bathroom lavatory is the only fixture in a residential structure that uses a 1-1/4" trap and fixture arm. The code will allow us to increase the size of the trap and/or fixture arm by one nominal trade size, which would then be 1-1/2". Rather than maintain a complete separate inventory of pipe and fittings for this limited use most plumbers now opt for the code exception and make the fixture arm 1-1/2". The code allows an 1-1/2" diameter horizontal waste line with a ¼" per foot pitch to convey a maximum of 3DFU’s therefore if your existing fixture arm is 1-1/2" you will be able to connect both sinks to the common fixture arm. If the inside diameter of the existing pipe is 1-1/4" you may not combine both sinks on a common fixture arm.

To make the physical connection you would need to use a 1-1/2"- "End Waste kit" or "Center waste kit" in the same manner as connecting a double kitchen sink. The lavatory drain tailpiece is 1-1/4" so you would need to use a 1-1/2"x 1-1/4" reducing ferrule on the inputs to the waste kit. The discharge from the end waste or center waste kit would then go into a 1-1/2" P-trap attached to the fixture arm.

For the water supply lines we are in a gray area of the code. The code requires all fixtures except a tub or shower to have a valve at the fixture. One exception to this rule is when we are running a manifold system such as commonly used with PEX. On a manifold system we have individual lines from the manifold to the fixture so we may use the manifold shutoff valve as the fixture shutoff providing the manifold valves are all indelibly labeled to indicate which fixture they control.

They make side outlet angle stop valves, which have the normal 3/8" outlet port on the top of the valve and a second 3/8" outlet port on the side. These types of valves are commonly used at the kitchen sink to connect dishwashers or ice maker lines. It would then be a judgment call by your inspector whether you could use a side outlet angle stop and run a line to each sink or whether you would need to install a tee on the water line and separate valves.