Invoice with multiple Membership lines
It is absolutely possible to have multiple line items on an invoice :) 

This can be done by using "Price sets". More information about this can be found in the book here:

http://docs.civicrm.org/user/en/4.6/membership/membership-price-sets/

Once you have set up your price set, persons (either administrators or public users who purchases memberships on a contribution page) could purchase 2 memberships in the same transaction. Then 2 memberships will then be shown as separate line items on the invoice.