[gentoo-user] [OT] Looking for a different way to rsync (or similiar)

Hi,

I have to old (technical identical) harddisks formerly used for backup
purposes. They contain each a different set of files -- the
partitioning is identical. Both disks are nearly filled.

The harddisk in my PC has more free space (they contain a totally
different system than the old disks!)

Rsyncing both disks against each other so they will conatin the same set of files
afterwards may fail, cause the free space is limited.

Is there a way to rsync old disk "A" against old disk "B" *BUT* store
the files in question on the disk in my PC?
(and do the same thing but with disk "A" and "B" reveresed?).

The result of this hassle should be one old disk containing all
(needed) files of the sum of "A" and "B" with not needed files
removed by hand *after* the rsyncing.

Or there a totally different way to acchieve this?

Thank you very much in advance for any help!

Best regards,
mcc