Actually, it’s quite effable. Tom gave the traditional statement quite clearly, and Joel has been amplifying it before and after with a well-thought-of link.
IIRC, Phil Dennison has a traditional technique for debunkery that involves taking a “defective” human – i.e., one with some ailment appropriate to the situation, and reviewing the argument in light of his/her (limited or nonexistent) options.
I want to reverse that for this post, for reasons that will become clear in a moment.
A human being is possessed of at most one personality. In cases of “multiple personality disorder” the function is that the person has several defective personae attempting to vie for control of his one body, and rarely if ever working cooperatively, except (so I am given to understand) in temporary alliances to defeat a third personality. In true schizophrenia, as opposed to MPD, the personality disintegrates.
Imagine a being with three distinct personalities cooperatively working together and loving each other, all in control because their will is unified (not one but directed at one end). In the “economic Trinity” as Joel notes, the analysis is directed at the functions of the three persons, each person playing a separate role vis-a-vis mankind. Eastern Orthodox theology gets into great depth regarding the internal functional relationships of these three persons; if it is desirable, I have an online Orthodox acquaintance I can ask to give some details of that concept here.
Because God is greater than man, He is able to manage three personalities at once, working them together like a centipede’s legs, so to speak. (Probably the first time in history for that analogy! :))
As for Jesus as god and man, the simplest explanation I know of is referred to as the kenosis (“emptying”) theory. Given the old conundrum of a possible rock that is so heavy God can’t lift it, the explanation being that He can choose to be unable to lift such a rock, denying Himself that ability, one features God the Son divesting himself of omnipotence and omniscience in order to be born a human baby, and live a human life as one of us. This kenosis is what He did, humbly, in order to feel what we feel, know only what we know, etc.
Does that cover it?