Your question is a bit confusing but ‘It’s been two weekends since we met.’ is ambiguous. It can mean both: ‘It’s been two weeks since we first met.’ or ‘It’s been two weeks since we last met.’ An alternative using ‘to live’ for ‘It’s been two weeks since he lived in this building.’ is ‘He hasn’t lived in this building for two weeks.’ (I feel that this is clearer)

Live is one of few words that are somewhat hard to put in tenses, i mean we *live* in the now and we lived it in the past, but for future the vocabulary will be *dead*--- 1: He has lived here for 2 years and still there. 2: Two weeks ago he/she/ it left. - Correct. If you say *it has been two weeks since he/she/it left* --- well continous left what?? Left a house? Apartment? Relationship? Date? Practice of some sport? Need to specify more here. If looking for someone it is usually in a negative sense, so people or even worst case scenario police needs to investigate further. *to live* - this we use in the moment!!!! We live in the moment.

Yes you can say it's been two weeks since he left.