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To make the dual ECU setup work, you would have to have some kind of trigger simulator to translate the engine speed of the hosted engine into what appears to be the 36-2-1 crank trigger wheel of the FA20. This would make the factory ECU believe that the engine is turning so that it would send the appropriate CAN packets out to switch on the PS, ABS and run the dashboard tach. If the host ECU uses an old style AC request pin to do AC idle up, then you may be able to feed the AC compressor request to that. You will have to do something to make sure that both ECUs see the right coolant temperature (probably install the FA20 coolant temp sensor into the new cooling system), or have the host ECU take over the function of the cooling fans. Even after all that, the factory ECU is going to quickly figure out that it really isn't in control of a real engine and various idiot lights are going to be on constantly. Also, the fuel gauge is always going to be reading incorrectly, since whoever designed this car had the bright idea to have the dashboard trust the ECU fuel consumption data on the CAN to adjust the fuel gauge.
You can certainly get the car to drive down the road with a dual-headed harness, but you will still have several annoying issues that will make the setup seem more like a science project than a great road car. Of course, if the entire idea is to build a pure race car, then who cares?
Most shops able to handle something like this will wisely choose the EMS route since that will be most in keeping with the principles of least surprise and keep the customer satisfied.
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