Sorry I said unionfs but it's actually handled by mergerfs [1], and it's all automatic. There are a whole boatload of policies [2] to control writes.

I use "existing path, least free space". Once a path is created, it keeps using it for new files in that path. If it runs out of space, it creates that same path on another drive. If the path exists on both drives for some reason, my rationale is this keeps most of the related files (same path) together on the same drive.

I see there's some newer "most shared path" options I don't remember that might even make more sense for me, so maybe that's something I'll change next time I need to touch it.

[1] https://github.com/trapexit/mergerfs

[2] https://github.com/trapexit/mergerfs#policy-descriptions