If you simply combine (via a Y-adapter) the signals from two microphones into one channel, the results may vary, but you are quite likely to compromise the sound quality severely.
In most specification sheets for microphones, there's a figure called "recommended load impedance" which is generally 1000 Ohms or higher. This represents the minimum value for that type of microphone; with lower load impedances some proportion of the signal will be lost, distortion will increase (especially at high sound pressure levels), and in extreme cases the microphone may not function properly or at all. The thing is, if you use a Y-adapter to combine the signals from two microphones, you're placing their outputs in parallel--each microphone then "sees" the other one's output as part of the load that it has to drive. And the output is generally a much lower impedance (typically 1/10 the recommended load or less). So you probably won't get a very good sounding result this way--and you could, in an extreme case, damage one or both microphones. Powering the microphones also becomes an issue.
There are two ways to get around these problems: "microphone mixers" and "microphone combiners". Both types of equipment have multiple inputs and (in general) some smaller number of outputs. When you plug two or more microphones into either type of equipment, the outputs of the microphones aren't connected together directly; resistors, transformers, and/or active circuitry are used so that each microphone "sees" the type of load it was designed to operate into. Mixers are more flexible because they let you set the amount of each microphone's signal that goes into the combined result--but most mixers contain active circuitry, and thus require their own power source (which may be a battery). There are, however, some purely passive mixers. A "combiner" usually uses a transformer with separate primary windings for its two microphone inputs, and a single secondary winding that drives the output. That's a completely passive arrangement, so no power supply is needed--but the only combiners that I know of are for microphones with professional-type XLR outputs, which isn't the type that you have, so someone else would have to recommend one if that's the way you want to go.
Apart from all these electrical issues there's an even more basic problem. Simply combining the outputs of two microphones, which are relatively close to one another in the recording venue, is a mixed blessing. Because different sound frequencies have different wavelengths, and both microphones are receiving the same sounds but at slightly differing times (because of their different distances from the sources), when their signals are combined, some frequencies will be reinforced while others will be diminished (partially canceled). The result can sound rather strange and artificial, and not what you want or expect; it's a game of chance. Most professionals avoid this type of situation as much as possible (see "3-to-1 rule" on the various Internets).
What a lot of people here seem to do is mix the output from the venue's sound board (if they can get at it) with their own recording. That's different because the balance and timing of the signals from various sources will be different from one another; each means of pickup could fill in something that was missing or unclear in the other.
There's also an effect that occurs when you have a s**t-ton of microphones in different places, all competing with one another to render the same sound sources: the mixture is so chaotic that no one conflict predominates in the resulting mix, since basically everything is in conflict with everything else. I don't generally like that kind of recording--but it can work in its own way, as long as there are enough separately-placed microphones to "swamp" the sound of the main stereo pair. At that point you can often remove the main pair from the mix entirely, and not notice much if any difference.
--best regards