Two accounts on one PC, and where the space actually goes

On a shared PC, Settings shows you what the system counts and gives you no way to see what another account is using. That is a privacy feature and it makes a full drive genuinely hard to diagnose.

Seeing the whole drive

# From an administrator PowerShell
Get-ChildItem C:\Users -Directory -Force -EA SilentlyContinue | ForEach-Object {
  $s = (Get-ChildItem $_.FullName -Recurse -Force -File -EA SilentlyContinue | Measure-Object Length -Sum).Sum
  [PSCustomObject]@{ GB = [math]::Round($s/1GB,2); Profile = $_.Name }
} | Sort-Object GB -Descending

That totals each profile without exposing what is inside, which is usually enough to know whose cleanup is needed.

What each account duplicates

Applications themselves are shared, which is the good news. Everything an application writes per user is duplicated.

Profiles nobody uses

The largest thing on a shared PC is usually an account that nobody has signed into for a year: a family member who moved out, or one created during setup and abandoned. Remove it through Settings, Accounts, which removes the profile folder with it, rather than deleting C:\Users\name by hand.

The Public folder

C:\Users\Public is readable and writable by everyone and is the sanctioned place for shared files. It does not deduplicate anything: a file copied from it into two profiles is two files. It is a convenience rather than a space saving.

Practical arrangements

1. Keep one media library in Public rather than one per person.
2. Check for accounts nobody uses, which is the single largest win.
3. Each person empties their own Recycle Bin; yours does not cover theirs.
4. Give each person the monthly routine rather than doing it for them.