Maybe not a good idea. Each mic will each be driving the low impedance output of the other mic. The mics are expecting to see something in the range of 1k to 50k input impedance and they'll probably be seeing something more like 200 ohms. If either mic is capacitively coupled, it will cause the low frequency cuttoff frequency to go up too. So, instead of having 50 Hz low frequency cuttoff, you might end up with 250 Hz or more. That would definitely cut out some audible signal at the low end. (It's not likely that your mics are capacitively coupled, so it is likely to be of no consequence to you.) The most likely thing is that if both mics are the same output impedance, the output of each will be reduced by 6 dB and summed for a net gain of 0dB (which is no change in level, relative to just using one mic). However, if they are both electret mics, they may not both want to be biased at the same voltage level, in which case you will probably see a fair amount of even harmonic distortion as the signal modulates the output impedance of the mic that ends up biased at a lower voltage than it would bias if used by itself. The bias voltage thing may change over temperature too.
Reader's Digest condensed version: It might work or it might not, depending on whether your mics are capacitively coupled, depending on whether or not both sets of mics want to bias up at the same voltage, depending on the ambient temperature and depending on how well the mics' output impedances are matched.