Hmmm . . .

Now, this would work depending on many things.

But I find, for me, that I get the best ideas for my work when I am writing another project.

So, if this was me, I would plan out both novels, then write one during NaNo and one during another month.

But it's totally up to you. The idea of doing 25k on each actually sounds good to me as well. You might learn more about each individual project when you are working on the other project. Maybe it's because the projects get jealous of each other, and that's how they show it.