If your are john and you want mail for both domains to go to you, then you have no problems. If there are 2 johns, then they will get each others mail depending on who pops theirs first. All mail for john@alldomains.com goes to /var/spool/mail/john. Right now the only way I can think of to get around it, is to create virtualusers and point them to unique user names or other e-mail addresses.
I am no expert on this issue but I host a few domains and have been trying to resolve this issue. I'm just passing on the information as I understand it.