moe
You can only have one joe in the system (as a user acct in this case), so for the second joe create a new user name eg joe2
Then add a pseudonym
joe@domain2.com forwarding to joe2
using the full email address, and then mail for joe@domain1.com and joe@domain2.com will go to the seperate joes.
PS It never hurts to read the manual as this is in there.