Design constraints when dealing with very high resistances in op amp's feedback loop
I used to do condenser microphone stuff professionally, which was very much this kind of thing (10G ohms with maybe a 30pF source).

Cleanliness is VITAL, a bit of skin oil in the wrong place and you will get weird popcorn noise, very annoying (An ultrasonic cleaner can be your friend here). 

I would absolutely advocate solder resist to keep the potential for surface contamination down, but really the way to go is to use a teflon standoff for the high impedance node, and think carefully about the possibility of using a discrete jfet for the input buffer.   

Consider guard tracks and also consider cutting a slot in the board under the resistor, extending the guard tracks to a pour on a layer under the resistor is also not a bad plan.

The problem with multiple resistors is they expose more places for contamination, and also give more capacitance to internal layers, neither of which is good.