I don’t know how common this is, but most of the people that I know who have two phones just kept their old one when they got a new one. The old one may not have cell service anymore, but still can use wifi. So they mostly use one for games or such, and the other for communication. Because phones tend to have their internal space filled up by just a few bloated apps and many apps insist on being on built-in memory instead of an SD card, one phone isn’t enough (despite the fact that they have way more memory than computers a decade ago and that SD slot).
I can understand having two phones due to the fact that they are so leaky with private info. Sadly it would legitimately make sense to have one for family, one for work, one for friends, and one for new contacts. They’re really not well-designed when it comes to things like that.