Union FS: getting more space from free cloud

Hello I'm trying to use rclone with the union function to achive this:
I create 2 mega.nz accounts each of 20G
"union" with rclone to get 40G
Backup my crypted data from synology hyperbackup to this "big" volume via webdav.

So the problem seems that the big 40G volume exist but I get an error when I write 20.1G (so it does not halt, let's say at 19G, and "jump" to the other free volume when needed)

So is it possible to use this "union" as decribed above? Some advice?

hello and welcome to the forum,

rclone cannot split a large file into parts and save each part of a different remote into a union.

tho might try https://rclone.org/chunker/
chunker -> union -> mega.nz