No, you only need a single constraint.  You can do it the way Thom showed you, with a computed (persisted) column, or you can put the whole calculation in the constraint itself:

CREATE TABLE Shipment (BoxID int IDENTITY(1,1),
   Width decimal(6,2),
   [Length] decimal(6,2));

ALTER TABLE Shipment ADD CONSTRAINT SizeRestriction CHECK (Width * [Length] <= 10);

John