Hi,
You are not specifying what type of the alert is but any of the alerts are specified either at resource. resource group or subscription. If you want to have the same alert on multiple subscriptions create the same alert on those subscriptions. If you are sending your monitoring data from multiple resources in different subscriptions to Log Analytics workspace you can create Log Alert scoped to the workspace and that way you will get alerted on any resource that sends data to the Log Analytics workspace.
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