OTOH, we never heard that the Borg would have taken control of the shields and sensors by using Tuvok's codes. All we heard was that the codes were compromised. This could just as well be because the assimilated Tuvok, familiar with the ship's security systems, found a way to compromise the codes that had been duly altered.
As for requiring more than one person to set self-destruct, I don't think this would be a good idea. Anybody with proper authorization should be able to set it solo, or else the ship's secrets could be compromised when key personnel were killed at the moment the enemy boarded the ship. It's merely a matter of deciding who should have the authority - and surely the skipper should! If she goes crazy or joins the enemy, that's her prerogative. She's the captain, she's fully entitled to life-and-death decisions without the need to consult anybody.
Timo Saloniemi