I'm fairly sure this is incorrect advice. AFAIK you cannot "upgrade" an award ticket (all references to upgrade on the VS website are from a paid fare class), you can only cancel an award and immediately re-book in the higher cabin. Therefore all of the miles must come from the same account. Otherwise people would game the system to combine miles from various accounts for free.

What you can do for sure is upgrade a paid ticket with miles from another member's account...