Sort User name by user's last name

I want to sort a list of user names by the last name.
The value of the username is firstname + lastname as entered by the user at registration in a single field.

I have a node whose title field is the name of a station of a volunteer event.
This same node also has a field with multiple values These multiple values are the volunteers assigned to the station.
This field is a user reference. The value displayed is the username.

I also have a profile with a firstname field and a lastname field that is associated with the user or account.

I can't figure how to display volunteers by their lastname with Views.

Any suggestions?

Thanks