Windows Server Backup with multiple devices

[woreilly]

Forgive any potential noobness with this query. I'm still in the learning phase of my job as an IT Technician. Anyway, I am configuring the backups for a school. They have 2 brand new QNAP TS-212 Devices and I am using Windows Server Backup to do the backing up. So far, I have setup a schedule and selected both devices as the destination disk(s). What I want to know is will it backup the same data to both devices or will it pick at random or pick only one device to do the backup? If the latter is the case, what would you suggest I do if I want to utilize both devices to their potential?