> Would the signal going to each of the two channels be half what it would be to a one channel?
With equal load impedance on the two inputs, the power to each would be cut in half--but the interface between a microphone and a preamp isn't based on power transfer; it's based on voltage. So there's no set rule; it would still depend on the specific impedances of the microphones and the recorder inputs. Still, impedances aren't always constant across the frequency spectrum, so the amount of the signal loss could vary at different frequencies, altering the balance of the frequencies in your recording. That's something you'd normally want to avoid.
Another issue that you didn't bring up is microphone powering, which can get messy in this type of situation.
You're doing something that professionals generally try to avoid doing. If you really need one microphone to feed two inputs, you could use a high-quality microphone "splitter" (basically a transformer with two independent secondary windings) but it would be better if you fed the signal into one channel of a good preamp (or into a good mono preamp such as a Sound Devices MP-1), then split the line output of the preamp. Otherwise, what you're doing would tend to reduce the quality of any recording that you make that way.
Just what do you hope to gain with this arrangement, anyway? Does your recorder only work in stereo, and it just bothers you to make a stereo recording with one channel unused? Or is there some other reason (e.g. you don't know what levels to expect, so you want to set one channel's gain 6 or 10 dB higher or lower than the other one)?
--best regards