Is it permissible to make up my Dhuhr and Asr prayers combined after the time for Maghrib has begun, if the prayer was missed due to travel?

You must pray the missed Dhuhr and Asr prayers immediately upon remembering them, and in their proper order, because of the Prophet's (peace and blessings of Allah be upon him) saying: "If one of you neglects a prayer or sleeps through it, let him pray it when he remembers it." And because of Allah's saying: "Indeed, I am Allah. There is no deity except Me, so worship Me and establish prayer for My remembrance." And since you are still traveling, pray them shortened.