When Load balancing is disabled, devices are statically assigned to objects and that can be changed in the Backup Object Summary tab. 
First make sure the needed devices have been enabled in the Destination tab.

Next, in the Backup Object Summary tab, the assignment can be modified. Either Right-Click the object and select "Change Device..." or select the object and click the button at the right.

The same can be done while creating the backup job initially.

Now, for a backup specification with Load Balancing enabled, the device assignment is dynamic and this "Change device..." option is disabled.

But in the Destination tab, there is now the possibility to define the order in which devices should get assigned. This order can be changed by Right-Clicking any of the selected devices.

You may be interested to some of my other Support Tips too: Support Tips listed per category.