First, せんせんげつ (the month before last) and にかげつまえ (two months ago) are not technically identical as the English words aren't, either. せんせんげつ refers to the month while にかげつまえ simply refers to how long ago it is=two months ago. Now today is Feb 7th, so せんせんげつ could refer to December (and it could include the duration from 1st to 31st or just one specific date or anything). にかげつまえ means December 7th (with a little range?). So, when a question is "What month was that? なんがつでしたか", then, the answer could be せんせんげつ + です/でした/だったと思います/etc. "How long ago was it どれくらいまえですか?" --> it could be にかげつまえ. (You can say せんせんげつ in this case, too. It never sounds unusual, but this answer does not strictly match the question) When a question is simply "When was that?", this answer could be possible にかげつ（くらい=about）まえのせんせんげつ　だったと思います or 6, 7 しゅうかん (くらい) まえの　せんせんげつだったと思います or something like that [I think it was (about) 6 or 7 weeks ago and the month before last]. Either one should be fine and にかげつまえ, にしゅうかんまえ may be rather common as the other people answered. However, I do often hear both (にかげつまえ、せんせんげつ). Especially in business/formal situations, I may answer following what I am asked.

I've never heard anybody say せんせんげつ although it sounds correct. にかげつまえ is more common. Same for せんせんしゅう.

せんせんしゅう＝先々週

せん＝before

にしゅうかんまえ＝二週間前

にしゅうかん＝two weeks

どちらも同じ意味ですが、

日常的には二週間前という言葉を使うことが多いです！

なんだか、少し伝え方のニュアンスが上手くて伝えられず、すみません