Nothing. The reason audiophile Schumann frequency generators work is because the natural one is too low in amplitude. So no, they wouldn’t cancel. What you should have asked is, if you have two Schumann frequency generators would they cancel each other? Or would they reinforce each other? Do you have to synchronize them?
For further study: how does a 25,000 mile wavelength signal fit into a room? Prize for first correct answer.
For further study: how does a 25,000 mile wavelength signal fit into a room? Prize for first correct answer.