Nothing sure, but two guesses.
1st: Michelotti as the designer of both. It happened with quite all the 600M vans, that are absolutely identical, made by Accossato, Caprera, Viotti or others: for such an utility vehicle Michelotti sold the right of the design to many coachbuilders. But this one is not so "utility" and Viotti was very proud of its "Giardinettas".
2nd: same sub-supplier. Viotti was big and sometimes had other minor coachbuilders as bodywork sub-suppliers. Perhaps Monterosa did the car for Viotti and then re-proposed it slightly modified with its own brand.
Another note: apart from the game, I'm quite sure that the word "Moncalieri" comes from a misunderstanding. It's the small town near Turin were Monterosa was based and has very little sense in Italian language as a name for a car. I think that on the back of a press-photo there could have been a caption "Monterosa Moncalieri" as the address and somebody has misreported it.
