The Find my iPhone app has nothing to do with it. You just need to activate Find my iPhone under Settings/iCloud. And whatever account you use to login to iCloud is used to deliver the one-time code. But you can configure the other account to deliver the code to your phone by SMS.But the thing is I have two Apple IDs:
1) iTunes and Apps. (which are used on the other family iDevices)
2) iCloud. (which is my personal info and the findmyiphone)
Let's call them IDcloud and IDapps.
A) If I do the two-step for the IDapps, I'm not sure if I should authenticate the FindMyiPhone app with IDapps, normally I'd use IDcloud for that. Or am I just logging into the FindMyiPhone app with that ID, just as I would log in with a friend's ID if they lost their iPhone?
2-factor auth is currently not used for accessing calendar, contacts or mail. It is only used if you purchase something from a new device or if you manage your Apple ID at appleid.apple.com.B) So you if you are using different IDs like me, you can't set up two step authentication for your iCloud data (like calendar, contacts, mail, etc.)?