When using two microphones to record a source, normally you will get the best results by placing the second mic three times the distance from the first mic that the first mic is from the source. This is known as the “3:1 Rule of Microphone Placement.”
An example: If the first mic is 1 foot from a source, the second mic should be placed 3 feet from the first mic.
While many engineers believe that adhering to the 3:1 Rule will minimize phase cancellation, technically this is not true. The 3:1 Rule works because the level of the signal entering the second mic (the one farther away) is reduced in level compared to the signal entering the first mic; tripling the distance substantially reduces the relative level of the signal in the two mics. This reduces the effects of phase cancellation, since the most cancellation will occur when the two mic signal levels are equal. In fact, if you crank up the gain on the second, more distant mic, you will still hear phase cancellation when it is added to the first mic.
Keep in mind that rules are meant to be broken; you may prefer the sound created by ignoring the 3:1 Rule — experiment and let your ears be your guide!