have you looked at dar I think it does what you are after, though I have not used it myself.
from the docs
Dar stands for Disk ARchive. From the beginning, it was designed to be able to split an archive over several pieces of removable media -- no matter how many or what size. Thus, dar is able to save over floppy disks, CD-Rs, DVD-Rs, CD-RWs, DVD-RWs, Zip disks, Jazz disks, etc. Dar is not concerned by un/mounting a removable medium; instead it operates independently of the hardware. Given the size, it will split the archive in several files (called slices), pausing before creating each new slice. This allows the user to un/mount a medium, burn the slice to a CD-R, send it by email (if your mail system does not allow huge file in emails, dar can help you here also). By default, (no size specified), dar will make only one slice. If a slice size is specified and dar creates multiple slices, the size of the first slice can be specified separately. This is useful if, for example, you want to fill up a partially filled disk before starting use of an empty one. At restoration time, dar will look for the slices it needs, asking for a slice only if it is missing and required.
Even when using compression, dar does not have to read the whole backup to extract one file. If you just want to restore one file from a huge backup, the process will be much faster than using tar. To extract one or more files, dar first reads the catalogue (i.e. the contents of the backup), then goes directly to the location of the saved files you want to restore, and proceeds with restoration. When using slices, dar will ask only for the slices containing the files to restore. You can also restore all files from an archive, in which case dar will read the slices sequentially. When doing a full restore, no slice (except the first and last slices) will be asked for more than once.