I just got mine today, and same problem no instructions. I have a WMR 100 which has both wind/humidity and temp/humidity sensors. I believe that the two cables on the STC800 are to power up these two separate sensors. So in your case, you only need to use one.
Not sure about setup. In looking at the unit it appears to have an internal battery pack. I did a quick voltage test under a light which says that this is so., in which case it should be a matter of charging in sunlight, plugging in, and doing the normal startupup/reset procedure. But I am guessing, and will try this weekend.
I have posted a request for answers to the Oregon Scientific web page, but feel free to contact me directly to see how things are going on my end.