There is no way to add alternate controllers within Alarm.com.
The ability to link Z-wave controllers is in the panel software, but it is untested and unsupported by Qolsys. Access to the add/remove controllers button under Z-wave requires you to enable advanced z-wave settings at the bottom of the standard Z-wave Settings menu. (Settings > Advanced Settings > Installation > Devices > Z-wave Devices > Z-wave Settings)
Here is a rundown of someone setting theirs up as a secondary controller: