Welcome to Less Wrong. We don't like having definition debates so I won't tell you how to use "atheist" but you should know that anytime someone uses the word atheist here they mean someone who assigns a very low probability to the existence of God, not someone who assigns a probability of zero. There has been some discussion here over whether or not 0 and 1 should even be considered probability densities. If you're interested I can link you to that discussion.

He is wrong, but it is entirely possible you are biased, from the Dawkin's School of Atheist Fundamentalism.

I don't know what you mean by this but Dawkins does not believe P(God)=0. And he is quite well respected in these parts. You may conclude from that that we are all irrational, but perhaps you should first investigate the possibility that you are wrong about Dawkins.

Is there actually any reason there can not be a Christian and an Atheist with an Equal level of Rationality?

Nearly all of us here are atheists so it makes sense to phrase the problem that way. Also, while an atheist could have enough irrational beliefs to be worse than a Christian (or other theist) the Christian has a huge head start. And if the atheist is an atheist for good reasons it is likely (but certainly far from guaranteed) he believes a number of other things for good reasons.

Once again, for emphasis, from the way this question is worded, neither is rational, and both are being depressingly biased.

The atheist is not ignoring the theists arguments for the existence of God. The atheist is ignoring the Christian's claim that he is too biased to evaluate the question correctly. The atheist has good reason to ignore this because the probability God doesn't exist is much higher than the probability he is too biased to get the right answer.