It's unlikely to make your vocals 'fat'. If the mics are very dissimilar, then you will get the 'good' and 'bad' parts of each mic. For example if Mic A has a resonant peak at 8 KHz and Mic B had a peak at 6KHz, you will no have peaks at both, and if there is a peak roll off between those frequencies, you could have another peak where the freq curves cross. Similarly, if both mics have a proximity effect and you are close to both, your bass response could be overwhelming when combining the two.
Remember that using 2 mics on a guitar serves a different purpose - micing different places on the guitar itself for the different tonal characteristics.