I was taught that one should never use both the cross cut and rip guides at the same time. The spacer that is used to determine the placement of the cut should be clamped to the rip guide close to the front edge of the table so that it is no longer engaged with the work piece when the cross cut guide pushes the piece over the blade. In this case I would make a two step spacer, one for each side of the dado, but clamp it so that it positions the workpiece on the cross cut guide well before crossing the blade.