backup and archive

We are trying to use one tape drive for the archive and the log backup. we wrote a script to do the backup log first, when this is done, do the archive. The probelm is that when to swap the tape from log tape to archive tape without physically checking to see if the log backup has finished (the light goes off when the backup finishes). It seems that the backup runs in the background, so we can't see the log backup status from the terminal. Any suggestions? Thanks in advance!