Two reactions that catalyze each other
An early synthesis of borazon from hexagonal boron nitride would be one possible example.  When other approaches did not work, Robert H. Wentorf Jr. added magnesium and voila!  The magnesium had reacted with some of the hexagonal nitride to form $\ce{Mg3N2}$, and the latter then catalyzed the hexagonal to cubic conversion.  (Today other methods have taken over.)