Otherwise this will degenerate into police being allowed to sneeze devices and operate them away from their owner's presence.
Make it possible to "lock in" a single biometric profile and not permit adding a second profile without automatically wiping all data.
1. they can be compelled by force,
2. they can be physically collected, unbeknownst to the owner,
3. they share all the risks of digital passwords, including being leaked,
4. they can't ever be changed, even when known to be compromised by 1, 2, or 3.
Much better to have 2+ passwords for deniable secrets.
A unlocks the device. Most people only have this one "normal" password.
B unlocks the device plus secret b, maybe some extra kinky porn so people feel they've found your real secret.
C unlocks the device plus secret c, your real secret, maybe Bitcoin wallets or that novel you've been working on forever.
You can go on forever with this stuff, especially if you have root on the device. Which gives you some clues about the true purpose of laws like this and who thinks they are useful.
Also make software self-destructing with a warning, i.e. if the user chooses it at installation time, all data will be destroyed automatically by the OS if they move the device off-premises. Make the setting unchangeable after installation time.
Police won't want to destroy evidence, so they'll have no choice but to leave it on premises.
I'm not trying to enable criminals, but rather enable whistleblowers and to not succumb to unreasonable new laws, and keep unethical searches for bad reasons in check.
This is not how forensic analysis works. Data is copied to read-only supports before any attempt of access is made.
(For twins they're more likely to have different usernames than different biometrics.)