Yes, you can set 2 AMDs in failover mode, feeding one CAS.
In the EUE console, you have to add those two AMDs on the list of devices.
Then open CAS configuration in the EUE console.
Choose to add AMD in 'data sources' section.
In the left column choose the AMD which will be main one.
In the right column enable 'use secondary data source' and choose the AMD which will be the failover one.
Each of those AMDs is unique, and CAS needs to send licens to both.
Based on above, you need license for 2 AMDs, in order to cover desired scenario.
If primary source failed then the available data source or secondary data source will be assigned to the report server automatically or we will have to assign manually as a data source.
Your explanation will be highly appreciable.