Smartphones should have optional extra user accounts, with no indication of whether they exist. Same phone number, but the account you use is the one that records the history. Keep one mostly clean and that's the one you unlock.
Android 5.0 does. It's actually quite hidden and I only found out about it by mistake when playing with my new update (although I remembered Google announced it earlier after that, but had forgotten about it).
More operating systems should be doing this, including desktop ones. Windows makes multiple accounts too obvious.
