When two mics pick up the same signal at similar levels and feed it to the same channel, phase cancellation can occur. This causes the signal to sound hollow, or just not-quite-right. This can be avoided in several ways, but one of the easiest is to follow the 3:1 rule. The distance from one mic to the next should be three times the distance of each mic to the signal source. More is better, but a 3:1 ratio is just about the bare minimum to avoid the phase cancellation effect.
You can read more here: Pro Sound Web post on mic techniques