Posted on a.net by someone who usually has good information from within Fronter:

Effective September 8th.

Midwest will be able to sell tickets to these cities using Frontier flights:
Albuquerque
Billings
Bozeman
Colorado Springs
Denver
Durango
LaGuardia (???)
Rapid City


Frontier will be able to sell tickets to these cities using Midwest flights:
Cleveland
Pittsburgh
Indianapolis
Washington
La Guardia
Milwaukee
Denver
Nashville
Dayton
Orlando
Fort Myers
Philadelphia

More to come on both directions from what I've heard.