Not sure how to implement this yet, but the quest Kill them all! (QID 11094) requires you to be friendly to exhaulted with Scryers, but also Revered with Netherwing faction before this quest is offered. The REP tag only allows for one rep.

In WoWPro_Dailies/Neutral/Ludo_NetherDrake.lua:

A Hail, Commander!|QID|11092|M|66.12,86.36|REP|Netherwing;1015;revered|N|From Taskmaster Varkule Dragonbreath.|
T Hail, Commander!|QID|11092|M|66.22,85.65|N|To Overlord Mor'ghor.|
A Kill Them All!|QID|11094|PRE|11092|M|66.22,85.65|REP|Scryers;934|N|From Overlord Mor'ghor, for Scryers.|

The PRE clause on 11094 makes it conditional on a quest that has the other faction requirement.
This would not have worked before 2.2.0, as I added the support for more tags into the dalies module.

Of course, this is just a hack that gets around this particular case. Are there any quests with pure dual reputation requirement that have no implicit prerequisite quests we can mooch off of?

That's a good work-around. I'm not aware of any, and for when it happens, that may solve things.

Closing as resolved, as the guide works!