Magical Girl GameJam #4 and Metroidvania Month 13 are running concurrently, start to end, so I thought of challenging myself to make a game for both. From the MM13 rules, there's nothing about making games exclusively for that jam, and I've seen double jam games before, but I noticed you cite Rule #33 in the main page so that surprised me.

It doesn't hurt to ask, though: Can I upload one game for both this jam and MM13? They start and end at the same time, so it's not as if I'm giving myself more time.

You should be good! (I might do the same actually, if I have time). I think rule 33 has more to do with jam-spam... there have been some submissions to previous MGGJs that were clearly submitting to as many jams as possible, and their games weren't really designed for them.

Down in the FAQs on the MGGJ4 frontpage it's clarified:

Can I submit my game into other jams?

• If it also fits that jam, yes. Feel free to do multiple jams in conjunction if it happens to coincide with MGGJ4.
  • Beware though if you submit to so many jams, it might look suspicious. If I see your game being submitted to jams that your game doesn't actually fit into, even if your game fits my jam, I might question whether or not your game fitting into my jam is just a coincidence.

Wellp I was late to check this but as Shibey clarified, perfectly yes! As long as your game fits both of them.

EDIT: To clarify, Rule #33a simply states that your game should be made for the sake of this jam. Basically you can make your game for the sake of multiple jams. As long as it is very clear that you have this jam in mind while making your game, it's all good!

Whoa I had the exact same question and I'm submitting to both jams too! Thanks for the elaboration