When you add a domain as an alias, rather than a secondary domain, you cannot edit anything before the @ sign. More information about this can be seen here on the Google Support site. An alias is just added automatically for everyone for the primary account. This is mostly used for setting up systems like dual delivery (among other things).
If you'd like to specifically add different @domain2.com addresses, you'll need to remove it as an alias and add it back as a secondary domain.
Hope this helps!