There is no advantage to combining front and rear. The front outputs and the rear outputs are the same on car stereos. For whichever unit you're feeding into the other unit, set the fader in the middle and use one pair of outputs to feed the other unit. Then the second unit does all the fading.
The only time this would get confusing is if the fronts and rears had some sort of different equalization and/or crossover associated with them. For instance, if your stereo had a "Front Image Enhancer". In that case, just make sure you turn off that feature.
The fader is not a factor in deciding which of the two units "comes first" in the audio daisy-chain. There are other factors, covered in
this FAQ entry.
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Tony Fabris