Allocation-Retention-Priority consists Three parameters
In case of Resource congestion at RAN side,bearer having high priority level can grab the resource of the lower priority level.
So,as per your example,if the default bearer UE1 has lower priority as compared to bearer of UE2,than UE2 bearer will grab the resource of UE1 bearer.
This value indicates that the service data flow or bearer is allowed to get resources that were already assigned to
another service data flow or bearer with a lower priority level.
This value indicates that the resources assigned to the service data flow or bearer can be pre-empted and
allocated to a service data flow or bearer with a higher priority level. This is the default value applicable if this
AVP is not supplied.
For details about this refer specs:ETSI TS 129 212 V8.14.0 (2012-01)