[Marylin]

Well, this particular topic is really generating a lot of debate. And to be quite honest with you I haven't seen anything in grammar saying that since and ago can't be used in the same sentence. It sounds a bit awkward but I hear that use very often. In fact, I am almost used to hearing it all the time. Of course, we tell everyone that since ties with "two years" and ago will take on "two years ago".
X noticed that the same question can be easily changed to:
He has lived in this flat for two months.
Plain and simple.
But what if the person wants to give the sentence an exact point in time ( since 2 months and not two months and 3 days, two months and 3 weeks, etc) and at the same time we know it happend in the past (ago) then how else can we go about it? How else would you paraphrase a sentence like that without spreading yourself too thin? Can you do it all in one sentence? To me it's a quick shortcut between Past Simple and Present Perfect Tense.
Perhaps it's too much to cram into one sentence but grammatical or not, it's very common nowadays.