It depends upon the horizontal distance between the center of the drains for the two sinks. If the CTC horizontal distance does not exceed 15" both sinks may be combined with a centerwaste or endwaste kit, then discharged into a common trap to one waste arm.

When installing a disposal instead of the common centerwaste or endwaste kits you should use a "Disposal Type" end waste or center waste kit.

A common center waste or end waste kit has a formed 1/4 bend (90deg turn) on the end of the cross tubes. If you use the 1/4 bend that comes supplied with the disposal then connect that to a common center waste or end waste kit often the trap will end up lower than the waste arm coming out of the wall.

With a Disposal type end waste or center waste kit one cross tube has a flanged end instead of the 1/4 bend. The 1/4 bend that comes with the disposal is discarded and the cross tube is connected directly to the disposal output. The will leave the cross tube about 3 to 4" higher and you will have clearance for the P-trap to the fixture arm.

The dishwasher is then connected to the dishwasher input on the throat of the disposal.

If the horizontal center to center distance between the two sinks is greater than 15" each sink requires a separat trap and drain arm.