Is it permissible to make up missed prayers while traveling by combining and shortening them, or is making them up only for when one is a resident?

Whoever has missed prayers and does not know their number should make up what will bring him certainty or strong presumption that he has fulfilled his obligation. The making up should be according to his capacity, without harming his body or livelihood. Some scholars say that one should make up two missed prayers with each obligatory prayer. If a traveler is making up prayers that he missed while not traveling, he should make them up in full.