Siri custom voice shortcuts support Linked Systems. Voice shortcuts can be set up for multiple systems that have been linked. For more information, see Set up Siri custom voice shortcuts.
Amazon Alexa only can be linked with one Alarm.com account, so if an account that has Linked Systems attached is used, only the primary account for those login credentials will respond to commands.
Note: Enterprise group accounts also cannot be linked with an Amazon Alexa system.