Adding Sub-out to switchbox ?

I've build a basic switchbox not a long time ago to be able to switch from 2 input to one out (my main amp, powering 2 speaker in stereo.)

Since my main amp (Aurex Sc335 MKII w/ his brother preamp Sy-335MKII) doesn't have a subwoofer out, I would like to add one to my switch box (2 'switchable' input to 1 output + 1 sub out). I'll eventually *try to* build a gainclone to power the subwoofer.

My question is, may I simply run a second pair of wires from my switch to a second out (either in series with the existing out or in //)? I'm not an expert in electric/electronic, and was wondering if doing so would weaken the signal and/or add noise.

thanks for your time.