The question itself denotes a perceived possible conflict between the two; if there is a perceived possible conflict, what desire which Christianity does not fulfill drives the question to be asked in the first place? Perhaps it's merely an academic inquiry in this particular instance, but I'm not pointing any fingers--I'm questioning the practical value of the question in general.
I used to say things like "I am a Christian, and a Buddhist", but then I realized, as Justin Martyr said, "All truth, wherever it is found, belongs to us as Christians." The point being that we, as Christians, should never feel a need to identify ourselves with anything other than Christ because, in Christ, "there is no lack"; that is, if there are any truths to be found in that particular set of events and beliefs we have dubbed 'Freemasonry', they are, of course, rooted in Christ, who is the totality of all truth; and if any differences are found, they are, for the Christian, merely falsehoods found intrinsic the practice of 'Freemasonry'.
Therefore, I ask again (perhaps in clearer wording), what does Christianity lack that would have a Christian wanting to also call themselves a Freemason?