You can have a Primary Coverage button for 8658 on 8665, which will ring whenever 8658 rings. Calls are still placed on the SA buttons of 8665. You can add this button at 8665 by going into the programing mode, pressing a button with lights, dialing *40 8658 and pressing enter.
You could also add Shared System Access buttons for 8658 on 8665 if you need to make calls out AS 8658, but you need WinSPM or use the console phone to do this.