but hey if 10L/10R is possible, this one shouldn't be that hard

Unforch, it's impossible - because it's hard coded into the DSP's function set.

I got round it by just connecting the Left F/R to my front amp, and Right F/R to my rear amp.

Totally freaks you out when you hear it as it puts left channel instruments at the front of the car and vice-versa.

But you do get 10F/10R

One of the few remaining Mk1 owners... #00015