After some time with one 166 radio box, I found the investment in a second really worthwhile.
I now have one configured as COM1/COM2
and the other as NAV1/NAV2.
They are set up inependently of the P8. I am sure you don't need to assign a P8 button to activate the GF166.
Taking the three buttons on each unit in turn from the left.
Box 1 LH button - toggles COM1/COM2
Box 1 Centre button switches active and standby frequency on selected COM1 or COM 2
Box 1 RH button - not used
Box 2 LH Button toggles NAV1/NAV2
Box 2 Centre button switches active and standby frequency on selected NAV1 or NAV2
Box 2 RH Button changes Box 2 to display the ADF frequency and the transponder. The RH button toggles between the two. The ADF frequency is set in the standby window and switched to the active window by the centre button. the transponder is set in the standby window, but does not switch to the active window.
To restore NAV1/NAV2 function, press the LH button.
I tinkered for ages with settings and this was the best I could come up with.
My P8 buttons are configured to display on a GF45
4 DME 1
5 DME 2
6 NAV1 Ident
7 NAV2 Ident
8 ADF Ident
This gives all the facilities necessary for radio navigation.
With one 166 box, I would have it set LH button to toggle COM1/NAV 1, and the RH button to toggle ADF/Transponder
I have had a RP48 and a GF46 sitting on the floor for about 3 months, as I just haven't had time to fit them. It means reorganising my instrument stack and making a new mount.