There are some situations where you’re instantly presented with a problem: that of multiple microphones. You might ask “Why is that a problem.” As you may have guessed, I have an answer.
Consider the situation where you have two non-professional singers standing RIGHT next to each other, and each wants their own mic. So, let’s say that you decide to use two SM58s on stands two feet apart (chosen because they have excellent handling noise rejection, good wind noise rejection, and are as durable as a lump of iron).
They go on stage, stand 4 feet from the mic (but still right next to each other), and now you are confronted with not only no separation between them, but also a really hollow sound with no distinction. Why? Well, consider what is happening. Each mic is pickup up both singers. Suppose Singer A sings, Mic 1 and Mic 2 both pick her up, but mic 2, beling slightly further away, ‘hears her’ slightly later than mic 1. Mic 1, is 4 ft. away from Singer A, while mic 2, being 2 ft from mic 1, is about 4.5 ft from Singer A (the hypotenuse of the right triangle). That extra half-foot represents about a half-millisecond in time, so mic 2 ‘hears’ that sound about a half-millisecond later. This creates in a “comb filter,” where some frequencies are reinforced, while others, in a repeating pattern, are canceled, and is the same technique used in a “flanger.” (remember that sounds travels at about 1 foot per millisecond). No matter what you do with those two mics, both singers are going to sound hollow. (like a flanger set with no modulation)
There are a couple of solutions: Probably the best is to move the mics apart. The rule of thumb is that when you use two (or more) mics in a situation like this, each mic should be AT LEAST 3 times the distance as the distance from the mic to the singer. So, in this case, where the singers insist on standing 4 feet away, you should place the mics twelve feet apart. Since the singers will undoubtedly comment on this, you should instruct them that they need to move closer to the mics. If they will sing 1 foot from the mic, the mics can be 3 feet apart. If they will actually sing INTO the mic (a radical concept for many amateur singers), they can stand closer together.
The other solution, which will take less coaching from you, is to give them one mic, and tell them to both sing into it. If they balance themselves reasonably well, this may be the best overall solution anyway.