Answer
(Fatwa: 952/952=M/1429)
You should perform obligatory salah of Isha behind Imam and avoid arranging separate jamaats in different floors of a mosque, though it is allowable to offer separate Tarawih in different floors of a mosque provided there is narrow place, no sound echoing, no chaotic situation etc, but it is undesirable (makrooh tanzihee). It is another matter if performed on account of little space, but it should not be made a custom. If you want to perform Tarawih separately, then also you must offer Isha salah with jamaat and choose any other suitable place out of a mosque for Tarawih.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband