The W and X are the left and right of the DB610 motor.
The WS and XS are the W and X with modifications to the supercharger. The rated altitudes are higher.
The Daimler-Benz DB 606, DB 610, and DB 613 aircraft engines were created by coupling two respective DB 601, DB 605, or DB 603 together. The large and heavy doppelmotoren proved troublesome.