If I understand correctly, your UD2 member names match the names of Entity members, and you want to surface the Entity descriptions for that of the UD2 members.
I can think of at least two obvious options.
Dynamically - Create a Dynamically Calculated member in UD8 that returns somthing like api.Members.GetMember(DimType.Entity.Id,api.Pov.UD2.MemberId).Description. You would surface it into your Cube View as U8#[YourEntityDescFromUD2Member]:V#Annotation
Metadata automation - wriite a Business Rule that copies that Entity Descriptions over to the matching UD2 members. You can choose to copy to the destination Description, a cultural description, or a Text[n] property. If copying to Descriptions, then simply display Member Descriptions for UD2. If copying to a Text[n], you would still need a Dynamic Calc member to surface the description as an Annotation.
