So, my identity records got mixed up due in part to the impedance mismatch between (first name, last name) and (first name, middle name, last name), resulting in my legal name suddenly changing until I could get the issue sorted out.

Two fields works in the sense that most people are going to be have something to put in them, but what that is and what they mean isn't always going to match what you intend. Either the system is going to be tolerant of such things (in which case you can probably get away with some variant of (display_name[, external_name]), or you're using it to interface with some other identity provider/consumer that requires a specific format, and that's what should be asked for.

Names are personal and hard to get right.