MSDN says:
A window station contains a clipboard, an atom table, and one or more desktop objects.
According to above, both of the winlogon desktop and the current desktop are belong to WinSta0; so they should share the same clipboard of WinSta0.
However, if you copy a piece of text in the current desktop, say "abcd", and press Ctrl + Alt + Del to switch to the winlogon desktop, then try to paste the text to the password box; you will not succeed!
Why?