To be clear, you can't have two PrCs in one level (which that build is doing) and you're not supposed to use dual progression classes (like FL) in gestalt either. So it's just a really illegal build.
Also, it's really about the same as Archivist 20 in terms of power (since they'd get all the same spells anyway, and you're actually one casting level down on one side and 4 down on the other).
Anyway, as for CR, the problem is that CR for classes doesn't work at all. It's listed as +1 per normal class level (+1/2 for NPC classes, IIRC), but that holds a basic assumption that all classes are the same. That's just not true at all, not even close. So, you'd have to judge it for yourself. Remember that NPCs are not as good as PCs due to lower wealth (if you give them PC wealth then when killed they'll give up far too much stuff), but as a baseline I'd say a somewhat optimized T3-4 class with NPC wealth should be roughly CR=level if played intelligently (after all, flexibility is less a concern with NPCs, who'll only attack in a single fight in general). Stronger tiers go up dramatically in threat level... a well played Wizard 12 could easily decimate many level 15+ parties. But gestalting really won't change things too much unless the classes synergize really well (Monk/Shou Disciple//Druid actually makes Druids a lot stronger, because their passive abilities work so well together), because in general gestalt adds versatility, not power.
In general, the strongest gestalt combos are ones where the classes synergize well but don't rob actions from each other. For example, Wizard//Sorcerer doesn't synergize well (they overlap, but their abilities don't help the other class, and their casting stats are different) and in general when you cast a Sorcerer spell you can't cast a Wizard spell too this round. Wizard//Archivist is better due to the stats lining up, but you're still robbing actions (unless it's a persistent spell based Archivist). Archivist//Factotum is amazing, because Factotum actually gives actions to the Archivist, and a lot of Factotum abilities don't actually require actions. Monk//Druid works well because Wild Shape+Flurry+Unarmed Strike all goes together without either class robbing actions from the other.
There are other resources besides actions that must be shared of course... feats are a big one, and gear matters too. This is the problem for Factotum in Gestalt... it's great on actions but Factotums are feat heavy. Druid works well in gestalt because they don't need many feats (Natural Spell is sufficient). One advantage of Archivist//Wizard is that both classes want the same feats, and usually items that help one help the other. Something really disjointed like Barbarian//Healer has trouble because the items and feats Barbarians want most (weapons to hit stuff with, offensive feats) don't help the Healer at all, and vice versa.
End result: there's no one good answer to the question of how much power Gestalt gives. You just have to eyeball it by comparing a given gestalt build to a normal build.
JaronK