Wellll, Gospel of John starts out "in the beginning was the Word and the Word was with God and was God" which is usually a reference to God the Son (aka Jesus Christ)..... so some people believe in a dual God but most christians adhere to the Trinity theory, believing that god is three in one and one in three, ...
of course, there has to be two or three, cuz as you point out, it says, "us", and obviously if Jesus was running around on earth for 33 years, he couldn't have vacated heaven and left everything to run itself, sooooo...
however, if you ask any Christian, "are you a monotheist?" they'll say "sure" (assuming they aren't in some backwater where they only use 1 syllable words....) but then you say, well, you have THREE gods, it would seem and then you are stuck w/ the notion that it's just one of those "unknowable" things that we humans (despite being made in god's image) just can't comprehend in this lifetime.
as a christian myself, this annoys the bejeebers outta me, cuz then you kinda get all mysterious and you might as well be a Zen Buddhist and sit around wondering about the sound of one hand clapping, etc...
hope that helps!