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I would be inclined to start them separately. This lowers the starting current and reduces the needed breaker size as well.

Is there any dynamic stability problem? Don't know. Once running one might mechanically lock them together. This might be important if a high inertia load was added.

It has been found in large auto plants that using multiple air compressors can be an energy saver. Much of the time full air capacity is not needed. In our case we have 1 5HP reciprocating unit that is normally used. Also have a 10 HP scroll type that is only used when more air is required. Seldom do we run both simultaneously.