Above all, do not put critical signals on the ends of the cable! In your case, that is pin 1 and pin 20. The wires on the ends of ribbon-like cables are the most prone to emitting and receiving EMI.
From a radiated emissions point of view, the worst thing you can do is put a clock signal on pin 1. Putting a sensitive signal on pin 20 is the second worst thing you can do.
Here's how I would do it:
Pin 1: Ground. Pin 2: Clk. Pin 3: Power. Then put a 0.1 uF decoupling cap at the connector (both ends) between Power and Ground. If you don't want power on the cable then use two ground pins, but still put a decoupling cap between power/gnd. Do the same things for the AFE signal, with power/ground on either side and a decoupling cap.
The next thing to do is properly terminate your signals. A signal that is perfectly terminated will not radiate any EMI. Of course we can't perfectly terminate anything, but the closer you get the better. Controlling the impedance of your PCB traces and cable is important since it goes hand-in-hand with properly terminating your signals. Do you even know the impedance of your cable or the traces on your PCB? It's probably time to find out!
All of this, from which wires on the cable to use, to proper termination, to controlling impedance all falls into the category of "signal integrity". This is a huge subject that we can't cover here. I highly suggest this book: High Speed Digital Design: A Handbook of Black Magic. It's expensive, but you will save every dollar by not having to respin your PCB an extra time or two.