I guess that they want people separate so they can compare stories if they had to. If people were entering a country on a so called holiday but were planning to work and the agent could ask the other adult.

I used to always go up with my husband, now they are very strict and we just go one at a time. I was just at Singapore Airport and the roving agent guy asked a family with 6 kids, or rather told them that each parent was to take 3 kids each in 2 groups.