Hi Todd, if I understand you right, you have the two separate mixes recorded (or being input) on tracks 7-8 and 9-10?
For mix 7-8:-
If so, you can simply roll up the Aux1 knob on Track 7-8 and the Aux1 knob in the monitoring section to output the mix from that track in MONO from the Aux1 jack to a headphone amp. Alternatively, instead of feeding a headphone amp from the Aux1 jack, plug the headphones directly into headphone jack 1 or 2 and select Aux1/2 instead of Main under the headphone volume knob. That'll give you the mix being sent to Aux1.
For mix 9-10:-
Similarly roll up the Aux2 knob on Track 9-10 and the Aux2 knob in the monitoring section to output the mix from that track in MONO from the Aux2 jack to a headphone amp. Note that you'll lose any internal effects when you plug a jack into Aux2 (it disables the effects).
The headphone jack acts differently with Aux2 - it seems to only output Aux2 to the phones, if something is plugged into the Aux2 jack.
It isn't ideal, but might get you the result.
Cheers, Rocky.