No. fbackup always rewinds the tape. Possible alternatives:

 (1) Stick with dump/cpio/tar/pax.
 (2) Use a pipe: instead of telling fbackup where the DAT is,
     let it send its output to stdout (-f -) and pipe it to the DAT, using
     Berkeley no-rewind device and dd with a suitable block size (e.g.,
     10K).  You'll lose fast-search and resync-after-error functionality,
     though.  Also, the complexities of managing multiple archives per
     tape make this a high-risk proposition.
 (3) Use NFS[R] and mount the disks of the machine without DAT to the
     other and back them both up there.  You'll have to mount 'em with
     root permissions and restoring a completely destroyed root disk will
     be messy.
 (4) Scream at HP until they fix fbackup. :-)