I have to tell you, that MK1+MK2, and MK Duo, are the same thing.
From
http://www.leedberg.com/mk/news/index.htm#duo:
MK Duo
(Last updated: October 27th, 1996) Acclaim has the rights to Mortal Kombat 1 and 2, and since they probably want to get as much money out of those two as they can, a logical move would be to put the two together. That's exactly what Acclaim is doing, in MK Duo for the Saturn and DOS CD-ROM. This game is going to be Mortal Kombat 1 and 2 on one disc, but it is unknown if the two will be mixed together (like MK Trilogy) or just put on seperately. If MK1 is seperate from MK2, this will be the first version of MK1 since the (ugh) Sega CD version. I just hope that Acclaim fixes the in-fight load time problems that dragged Saturn MK2 down so much. If they do, this game does have potential.
I was also able to find something in the Official Sega Saturn Magazine, Issue #10 (August 1996):
But this also shows MK1 and MKII as two separate games, not combined into one. I think you are misremembering what MK Duo was.