Last month, Apple announced a feature called iPhone Handoff. Not to be confused with that Handoff; this is a new one.
iPhone Handoff allows you to keep your number active on two iPhones at the same time so you can easily switch between them. According to Apple's support website, you pick one phone as your main phone and set up Handoff on the other as your secondary phone. When you unlock one of them, your number becomes active on that phone - that's where you'll get calls, and that device will be used for Find My.
The idea is that your secondary phone will be something you pick up on weekends, for example, with apps and settings configured …