I appreciate the responses. I was curious because I thought to use it as a metaphor, in the same spirit as the medieval doctrine of the two swords (church and state). I was looking not to use that because the associations are too close and I thought of archery. Seemed like the two hands working together to fire a weapon was a usable metaphor. I've also considered something like helm and shield.
In Altearth, it's not church and state, it's the lay lords and the mages. Two power centers. A character wants to have the two work together. Hand in glove is too trite and, as I said, two swords runs too close to the historical precedent. Anyway, that's why I asked.
Just in case anyone knows German or French or Italian or Spanish (or Dutch, Polish, etc.!), I'd still be delighted to hear if there are specific terms (though not surprised if there isn't).