Is it permissible to have two jama’ats for salah at the same time in the same masjid?
It is impermissible and the sin will lie on both congregations and at least one person from them. If there is one Imam who is pious and has the correct beliefs whilst the other is a fasiq and holds deviant beliefs, then the sin is on the latter. If both are pious and have sound beliefs, then there is no fault on the one who is first to say the Takbir (and start the congregation). If both start it together, then both shall be sinful.
And Allah ﷻ knows the best
Imam Ahmad Rida Khan رحمة الله تعالى عليه
Translation by Dr Musharraf Hussain Al-Azhari Translator of Majestic Quran, www.majesticquran.co.uk