I don't is this will help but I'd rather have 1 1DX than 2 5D MKIIIs.
Yeah, I mean, why have two separate cameras when one will do the job of both models? You can only take one photo at a time .
I wouldn't leave the studio to go to a location job with only one camera body. Ever. Ever
Your current combination of a 5D2 and a 1D4 was until a few months ago THE
stellar pair of bodies.
While the 1DX is unquestionably a strong, tangible upgrade, any self respecting professional would have your current setup over just one 1DX body.
I say keep 'em.