|There is not a lot you can do about this as it is down to the way your ISP has its mail server configured.|
You have one email accounts and two email addresses associated with that account but with different aliases.
When each of you log on, you are both logging on to the same account and therefore get all the emails belonging to that account regardless of alias it uses.
You need to contact your ISP and check if there is more than one Email account available. Some ISPs do supply more than one Email account, some charge extra for the privilege.