I'm afraid that you'll need one column in the list  for each person you
want to assign  it to. To make matters worse, the email functionality in
the Issue's list will only work with the default Assigned To column.

Some ways to work around this would be to assign  the issue  to a generic
AD account (if you use them) and then any user with access to that
generic account will see the email notifications.

You can also add another column to the list  for the secondary assignment
(can use a Lookup type for consistent names), and instruct the users on
how to set up a filtered alert based on parameters that include their
name.