I think one easy way (if this is aceptable) is to use subdomain in the address, like this user@sub.domene.com In this way each server will just have its own subdomain.
I think one other alternative is to have one server as "mailfront" and then to forward from "mailfront" to servers with subdomain adresses. Don't think this will work very good as acounts have to be set up and maintained at two servers.
Don't know to much about this. Possibly somebody else has a bether answer.