Possible to union several mounted hdds similar to unionfs?

Hello, I have a powered HDD enclosure with 4 disks attached to my rt3200, and have each of those mounted to separate folders within my /mnt folder (/usb-meda1, /usb-media2, etc...).

Within each of those are folders with media (so in usb-media1 is "Movie Downloads (1)", in usb-media2 is "Movie Downloads (2)", etc...).

Is there a way I can union the different folders I've mounted so that I can view the overall contents of each folder merged in a separate folder? In openmediavault I've this is done with unionfs, and I see in openwrt there is fuse-overlayfs, can that be used to achieve the same effect?

I see in the overlayfs documentation they show an example command:

fuse-overlayfs -o lowerdir=lowerdir/a:lowerdir/b,upperdir=up,workdir=workdir merged

But it's not clear to me what the input directories are, the output ones, what lowerdir vs upperdir are, etc. So I wasn't sure if it made sense to use for my situation. Thanks for any ideas!