Did you ever hear back on this?
Running into the same issue here.
Basic Alerts are tied to each Polling engine and have to be modified on each Polling Engine separately. Basic Alerts are contained in the Alerts Table and have EngineID to specify which Polling Engine they are assigned to. You would need to make a copy for each Basic Alert to assign them to every polling engine, then assign the EngineID to corresponding Poller out of the Engines Table.
Advanced Alerts are designed in a way that instead of checking the Services on each system, they perform SQL Queries against the database and apply to all polling data regardless of the polling engines. Advanced Alerts can also only be configured on the Main Polling Engine as this is the only engine that contains the Alerting Service.