Backups not using all drives

I'm sure this is simple, I just setup a new drive on my jukebox, it works, I can do backups with it, I now have a Drive0 and Drive1. Sometimes the backups run on Drive0 and sometimes Drive1. I was under the impression that once I added this other drive it would start backing up different sessions or save sets/servers at the same time between the 2 drives. So While Drive0 backs up ServerA for example, Drive1 backs up ServerB. Is there something I could be doing wrong? I've tried playing around with the parallelisim and set it to 1 and even tried 2...but either way it didn't work.