[Rail Jon Rogut]

The Master output fader isn't an assignment.. it signifies which output the fader is controlling. By definition, a master fader is the final master output for the defined output. What you want to do is instead of using Outputs 1&2 as your mix buss.. create a stereo Aux track, assign the inputs to an unused stereo buss pair (say Busses 1&2) -- assign the outputs of the Aux track to Output 1&2 and while holding down the Control key on the Mac.. also assign it to any other outputs.. in this case Outputs 7&8. For your mix tracks, assign them all to the same Buss pair you selected for your Aux track input.. in this example Busses 1&2.

Rail