A back to back corner is just like cutting 2 - 90 degree outside corners.
Just measure the distance between them before cutting.

How to cut foam crown molding back to back corner

 Crown Molding Solutions offers inexpensive Flexible crown moldings, Foam crown moldings.
Ornate crown mouldings, Polyurethane crown moldings, Ceiling medallions,
Floor base moldings, Chair rail moldings and Installation products.
909 North Nolan River Road Suite E, Cleburne, Texas 76033
800 315 6311     Fax 817 641 6711

 

aaaaaaaaaaaaiii