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.
It is not crazy; it is efficient. 30 seconds in difference could not possibly make film thin or overcooked unless the developing time was very short (under 5 minutes). Something else ruined your film. Not metering/exposing properly, using a mis-calibrated thermometer, not testing your film to find it's optimal development first, etc. There is no logical reason that developing for 11:30 will make or break negatives that are "supposed" to be developed for 11 and 12 minutes.
"Truth and love are my law and worship. Form and conscience are my manifestation and guide. Nature and peace are my shelter and companions. Order is my attitude. Beauty and perfection are my attack."