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This guide will show you how to create an Alarm.com Login (which is used to access the Alarm.com account and is distinct from a User in Alarm.com who receives notifications and has panel/lock access)
If you want that new Login to be able to access both Alarm.com accounts, you will need to set the new login up using the Link a System feature. This way the new ADC Login can have access to both accounts and not have to logout/in when switching between the two accounts. This guide here covers how to Link Multiple Alarm.com accounts