Not at all uncommon back then. My company had salesmen and field engineers who covered many countries including Israel. The would carry separate passports so when they went to an Arab country, they wouldn’t see and Israel stamp on it.
I bet your colleagues didn’t carry Austrian passports under fake names...
I believe that was also common between Turkey and Greece.
Normal occurrence, while working in Arab countries, if you traveled to Israel, you had to go to Crete and get a second passport. If you are traveling on business around Africa and the Middle East, it is common sense for a number of reasons. In most countries intelligence services monitor people’s coming and going. They also monitor where business people have traveled recently. In third world countries it was not a bad idea to use a separate passport which provided little useful intelligence. Today, even a third world cesspool can track you via the internet. Multiple passports a spy does not make. Life isn’t a Netflix movie.