The Cortex provides you the option to use 2 Spektrum receivers but they aren't required. I could see them being useful in something like a simple sailplane that only needed 2, Ailerons, 2, elevators and a rudder. If you plugged in 1 or 2 Spectrum remotes in such a setup, no external receiver would be required.

If you're using an external receiver, simply don't add any Spektrum remotes to the Cortex. Just follow the setup guide and, for example, connect your right aileron to input channel A of the Cortex, and then plug your right aileron servo into servo port A of the Cortex. Just do that for all the channels you wish to stabilize.

Personally, I just installed a Cortex in the Skmaster small Hawk that I just completed. I added a JR G370A for the nose wheel steering. I considered using a Y harness or a Matchbox to match it to the rudder but I also wanted to be able to have a separate gain control for the nose wheel and to be able to disable nose wheel steering while the gear is retracted. Since I already had the JR gyro on hand it just seemed to be the simplest solution.

As for Dual Rates and Expo, yes you can still use them you will probably want to cut the expo in half or possibly eliminate it completely when the Cortex is enabled. On my other plane that I tested the Cortex in, enabling it made the controls feel like there was LOTS of expos. I have my transmitter setup so that enabling the Cortex automatically cuts the expo in half on all controls. I'll fine tune it from there after test flying the new plane later this week.

Also, when doing the setup procedure, make sure your dual rates are at the highest setting that will be needed for this airplane and that nothing binds at this setting as the Cortex uses these to determine it's travel limits when making corrections.