Yes, it worked by mounting both USB disks and configuring a “local” remote (Local Filesystem) in rclone.
Afterwards, I could configure a “union” remote (Union) which mentioned the “local” remote twice with different paths pointing to both mount points of the USB disks.
When configuring the “union” remote, I chose non-default placement strategies for “create” and “action” as I wanted to spread load evenly and path preservation is not exactly working that great, if you act on restic repositories instead of a plain filesystem.
Anyway, once I figured everything out it was working, but I still decided to just split my backups manually. In case of a drive failure, I only loose parts of my backups and the speed is also better that way. Partly, due to less overhead of rclone, but in my case also because I had a Seagate Barracuda (SMR drive and therefore not great on massive writes) combined with a Seagate IronWolf Pro (CMR drive which is way faster on the inital backups and provides consistent speed in comparison).
Additionally, partitioning my data manually is a conceptually easier architecture and in case of a disaster it would be way easier to restore as I don’t need to remember/explain any rclone config.
Finally, as I had two 8TB drives the need to split datasets was okay, I might have decided differently, if I would have needed more complex logic to split my data manually.