The "fake" user/profile should work like a duress pin with addition of deniability. So as soon as you log in to the second profile all the space becomes free. Just by logging in you would delete the encryption key of the other profile. The actual metadata that show what is free or not were encrypted in the locked profile. Now gone.

The same way when you buy a brand new phone with 200GB of storage that only has 50GB free on it haha

"Idunno copper, I'm a journalist not a geek"

System files officer ;)