Praise be to Allah.
What is required is to offer the prayers at the proper times, as Allaah has enjoined. Allaah, may He be glorified, says (interpretation of the meaning):
“Verily, As‑Salaah (the prayer) is enjoined on the believers at fixed hours”
It is not permissible to join two prayers except for an excuse that dictates that, such as travel, rain or sickness. What you have mentioned of being able to pray in congregation is not an excuse that makes it permissible to join prayers.
Ibn Abi Shaybah (2/346) narrated that Abu Moosa al-Ash’ari and ‘Umar ibn al-Khattaab (may Allaah be pleased with them) said: Joining two prayers without an excuse is a major sin.
Based on that, you should pray Maghrib at the proper time, then when the time for ‘Isha’ begins, everyone who is near a mosque should pray it in the mosque, and those who are far from the mosque may pray it with his brothers, if anyone is around, otherwise he may pray alone, and he excused in that case.
And Allaah knows best.