The 645 acting as a AP would only ensure maximum coverage and stability working in concert with the 655. You can do vs versa too.
For my set up, I use the 655 as the host router at one end of the house and another AP towards the other end in the living room.
Placement is crucial if your going to have the 655 be the only item servicing WiFi. Central is best. However be aware that other things will impede signal coverage, building materials and external interferences.
How big is your house? Sq ft?