What is the term for the combination of two keys at the same time?

The term that specifically refers to the combination of two keys played simultaneously is bitonality. This concept involves the use of two different tonal centers appearing together, which can create complex and rich harmonic textures.

In musical settings, bitonality allows for the exploration of contrasting key areas, often leading to unique soundscapes that challenge traditional harmonic norms. For example, one hand might play in C major while the other plays in G major, producing an intriguing interplay of consonance and dissonance.

The other options represent different musical ideas. Polytonality involves the use of more than two keys simultaneously, which is broader than bitonality. Atonality refers to music that lacks a tonal center altogether, eliminating the sense of key. Modes are scales derived from the major and minor scales that provide different tonalities, but they do not imply simultaneous keys. Thus, bitonality is the precise term for the scenario described in the question.