Uh, it WAS originally on DVD-18, but the demand was too great and the replication plants couldn't produce enough of the DVD-18 versions so a few 2-disc editions were unceremoniously released alongside the DVD-18's. I never heard of anyone getting the original 2-disc version, I believe it only made up a small number of the original print run. Artisan eventually re-released T2:UE as a 2-disc set in a double alpha with a cardboard slipcover.
The DVD-18 version is a doozy. If I had to take a solitary disc to a desert island (one with power and a DVD player and a TV obviously ) it'd be that one. Before T2:UE I had Artisan's original R1 DVD of T2, and I also bought T2:Extreme for the gorgeous new transfer. I even prefer it to the DVHS version (now sold on) which used the old Ultimate Edition HD master. Yet even with a mediocre transfer, the DVD-18 Ultimate Edition is still one of the greatest discs ever produced IMO.