I have found a lot of value recently to putting multiple microphones on a source in order to get a better sound for the mix. I am not talking about stereo miking a source for width, but blending two together as a mono signal. There have been a few times where I thought I had a great sound, only to blend in a second mic to find that I like it even more.
In a home studio, when often you are working with a makeshift space, it can often be a challenge to get a good sound with just one microphone, I understand. My room is not very big, which poses various problems acoustically. I am not proposing a solution to this, you have to start by getting one microphone in the right place and chain so that is sounding great on its own. Take the time and get it right at the source and you will be happy you did later.
After setting up my main microphone, I then choose a microphone that I know sounds different, often something that will sound darker like a ribbon microphone, and put it in a different position. This could be anything from a foot away from the first mic, to behind the source, to positioning it as a room microphone. Listen to your room and find a position thats sounds nice in the room, and might add to the song. Always be aware of phase issues. If you aren't careful, another microphone will do nothing but hurt the sound, then it will be a waste. Unless I am looking for something specific, I don't spend tons of time on the second microphone, since I know my source already sounds good. I think of it as an added bonus that might create something I like in the mix.
Some applications that have worked for me:
1-2 feet from the main mic on Acoustic Guitar
Acoustic guitars are such a rich, harmonic laden instrument, and the sound comes from the entire length of the strings. Point a second microphone at a different part of the strings for a second flavor of sound to blend in. Pointing one at the fretboard can also get more natural fret noise- if you like that sort of thing. I will often use a matched pair on acoustic to get the real song of the guitar, but a different microphone will bring out different tones and harmonics.
On the Rear of a Guitar Amp
Depending on the style of guitar playing you are recording, you can use almost any type of mic on a guitar amp. Standard form is to position it in front of the speaker, and it usually sounds great. A few times when I was recording a richer guitar part, like a cleaner hollow-body I decided to put a large-diaphragm condenser behind or to the side of the amp.
At the feet of a Piano Player
This can help pickup more low end, which may be desirable depending on the song. It will also pickup more hammer sounds, and well as the pedal and dampening sounds. These little noises can go a long way to making a piano sound more real.
Take a secondary microphone and put it in a random spot in (or out of) your room. Face it at the source or away from the source. Put it a few feet away, on the other side of the room, or down the hall. Any of these options can make for an added color that you may choose to add into the mix. Try it especially on loud sources such as drums or guitar amps to get natural reverb from your room.
Put two different style micrphones right next to each other on a vocal. Especially if you are can't seem to find a microphone that gives you what you want, you can blend two in order to get more options. Try using a dynamic and condenser side by side.
As I get more experience I hope to try this on more sources. Experiment and have fun with it. You may just love the results.