بسم الله الرحمن الرحيم
(Fatwa: 151/137/D=1431)
When you were sahib tartib (i.e. you have no salah left to be performed after puberty), you should first perform Maghrib and then Isha. But, if you are not sahib tartib you can perform Isha timely during journey and can delay performing Maghrib until you return to your place.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband
