You have two options.
- Doing what you said is perfectly fine in general. Some jams specifically say "no crossposting" but as long as neither jam specifies that in the rules, it isn't an issue. Mixed Media Queer VN Jam has no rules against crossposting.
- Second option is to make the game within the overlap of both jams, and then use the extra time of the longer jam to make an update of the game that has more content/polish. Some jams have rules against making updates while judging is in progress, but for the sake of MMQVNJ, it is fine to make an update as long as you keep a version of the game as it was originally posted to MMQVNJ available for download until judging concludes.
If my explanation didn't make sense or if you have any other questions, please feel free to ask!