Your best bet is to buy a stereo microphone. That is what it is designed to work with. splitting the single jack in two may give you a very low line volume, and may even ruin the sound quality.The whole point of this unit is surround sound, or a least stereo, so to use a mono mic setup would kind of defeat the object here.
The stereo microphone's have, essentially, two heads, one taking in the left, and one for the right. The result is quite something, as it will immerse you in a stereophonic world (especially using headphones!)
I don't know your budget, or what you will use this setup for, but with a bit of searching, you will find a mic that fits your needs.
Or, visit an audio shop and test some! Microphones vary vastly in there differences, so go and check some out.
Good luck, and hope this helps!