Am in Sao Paulo transit now so I thought to post this while it is fresh. Arrive on BA from London - gate 11 - on the tip of a pier in Terminal 1. As we got to the top of the jetway there was a sign saying arrivals and most people were turning left to follow it. But straight ahead there was an open door into the departures area. There were 2 BA staff and one pointed to the door and said transit every so often. Two steps past her and we met the other staff member who asked our names, looked us up on a list and let us through.
So far so good. Despite being on separate tickets LHR/GRU and GRU/SCL the staff at LHR had through checked the bags and given us boarding cards (one of the oneWorld benefits). We looked on the departure screens but no mention of a LAN flight to Santiago. The boarding pass says gate 26 so we started following signs to gates 14-27. Just as yuou get to the immigration booths, round behind some stairs, up an escalator there's a walkway (with travelators) connecting T1 and T2. At the far end we found the AA lounge (we could have used TAM instead). Next step is to go down the stairs, into T2 and find gate 26 (assuming they don't change it!).