I just discovered the Godot Engine and want to practice a little bit. However I'd also like to have a go at the Quarantine Jam which is in the same time frame. Is it possible or recommended that I join two Jams and when the Quarantine Jam is over I start on the Godot Jam, as the Godot Jam lasts longer?

Better make one very good game than 2 half-good games.

You can be more ambitious when you have more experience and know your limits.