I've done something similar once but had the security of 24+ hour layovers in FRA both directions. I'd say that reduces what risk of misconnect by a great deal, but as others have said if it goes wrong, it can go REALLY wrong.

I'm doing a UA/JJ (TAM) itinerary later this year, and even if cheaper to do separately, I will not risk it and even such have built in a three-hour layover in GRU. Both airlines run the high risk of delays in and out of GRU.