Sometimes the 2 headgasket deal was a tactic used to make for a streetable compression ratio for normal use but allowed the racers to to increase compression ratio by using 1 and still meeting factory specs.The factory specs. called for a headgasket of a certain thickness but rarely specified how many were used,so if they were torn down they were legal while maximizing their compression ratio.As skydogs post points out you had dig into the service bulletins to find that 2 headgaskets were used.Fords version of this for the 427 was that they had 2 steel shim gaskets with 2 part numbers,the .015 gasket and its part number was the one specified in the factory specs,and this was the one used in factory race cars like lightweight Galaxies and Thunderbolts,but the street version 427 cars had the .030 thick gasket installed from the factory.Its part number was the one specified for use in cars in for service when they were to be used for normal street duty to avoid complaints from use with lower grade fuels.