I have tried "crazy" stuff like this... something always turns out "thin" or "overcooked"... Images taken arn't worth the risks of this sort of thing.

What I HAVE done and it has obvious risks is add a rool wound on a reel "later" to the mix, I agree if things are in the ballpark and it's photojournalism deadline time do it (as I have) otherwise it is a false economy to do this on a basis of cost.... the economies of time are another story.

I have done this when I have a couple rolls that are being "pushed" with a couple rolls normal, the hazzaards of fogging the film or mixing up the pour in the dark are issues too.