RDX drives are supported by Acronis Backup 12.5 as a backup destination. They are treated as a regular local backup location.
If you are backing up to a shared RDX drive with swapped disks, set up automatic cleanup via a command or script, so that the disk content is erased before backup starts. See Acronis Backup 12.5 and Acronis Backup Cloud: Backing Up to Removable Devices with Regular Drive Swaps This way each disk will contain one full backup.
(!) Note that rotating removable drives that you use as backup destination must have the same drive letter assigned to them.