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The main reason was to reduce the ability of a single man to raise an army and wield it against the Tetrarchs. An army commander had troops (and possibly a lot of them) but did not have the infrastructure to keep them fed and supplied if he revolted. Similarly, a disaffected governor had no troops to raise a revolt. So an internal revolt had to rely on the coordination of at least two disaffected men in the same area, and this gave a much smaller chance to happen and a much higher chance for discovery while the two planned than the previous method, where one man had wide civil and miliary powers at the same time.