Personally, as much as I would prefer that blatant spam not be allowed to exist on any site any longer than it has to, I think we should not allow an exception for this type of post.
Some reasons to consider:
Simple and absolute rules are easier to enforce
If some specific types of collusion are allowed, then that makes all sock puppet based investigations more difficult. Currently, if the same user's multiple accounts interact or perform actions collaboratively in a way that would not be possible by a single user, then it is in violation of the rules. Adding exceptions to this would require a more nuanced look which increases the time needed to review and the chance for mistakes.
Effective Moderator Powers
Because six red flags on a post will automatically delete it. If a single user can cast two flags then this lowers the total number of users needed to remove the post. This is an abuse of the system.
Beyond this, if two is okay, how about three? Or four? Would it be okay if someone used six accounts to nuke a post as long as it is "blatant" spam?
Or what if the limit is two accounts per user, but this is an accepted practice? In that case, three users on two accounts each could also nuke a post with half of the set number of users needed.
Conflicts with the Existing System
The Charcoal project allows users to sign up to auto flag posts based on some conditions. This can occasionally result in four flags instantly placed on the post without direct human input. In this case, if a user is able to post two red flags, the post could be removed with only a single user manually evaluating it.