The MX-2000 unsingle was designed as a single barreled gun. You cannot buy a set of O/U barrels that matches in comb height. That may or not be a deal breaker for you.
The MX-2000RS is the exact same gun with a slightly higher adjustable rib. It can be ordered as a combo, or convert to one later. The MX-2000RS O/U has an adjustable rib and the rib heights are the same.
An MX-10RS is the same gun without the engraving on the receiver, and the bluing is not as nice. The MX-2000RS O/U and the MX-10RS O/U are identical except for the markings on the monoblock and the quality of the bluing.
Today, the MX-2000RS is considered a more desirable gun and fetches a premium (used) over the MX-2000.