Chris
You can backup to different devices at the same time as long as you backup different sources. It is not recommended to backup the same data twice at the same time (for example, backing up a machine from two different media servers simultaneously or with one media server to two different devices). If the sources are different, there is no problem, if not, you can have trouble.
What you can do is backup to disk and, when it finishes, duplicate to tape.
Regards