Basically, if two mics pick up out of phase frequencies from an instrument, they are going to cancel each other out. The result when you have two mics that are out of phase is that some frequencies are going yo cancel each others while others are going to add each others, so your frequency reponse is going to be really distorted, and the sound is unnatural. Usually, the best way to check is just to listen: try inverting the phase on one of the mics (most if not all plug-ins in PTLE have a button to invert the phase, the striken O). Usually, one of the setting is going to sound natural and the other not. Choose accordingly.