Just because a head gasket leaks externally doesn't mean it will leak into a cylinder. If it leaks to both, you have a major failure like the head warped or cracked. Usually when a gasket leaks externally the gasket is deteriorating and will only get worse. A temporary fix would be to get some GM cooling system pellets that they use on all of the aluminum block engines. I've seen them stop a pretty major leaks. If you were running coolant through the engine, it would be fairly obvious. The smell and the over pressurized radiator would be dead give aways.
Good luck,
Scott
