In the Fall of Five we learn that
Five was working with the Mogs since about a year after he came to Earth.
If so, why didn't they break the spell by bringing him and Six together when they had captured Six?
I don't believe that they knew the extent to which the charm worked. They knew that there was a charm protecting the Garde and soon realised that they had to kill in a specific order but i dont think they knew how to break the charm.
In Five's Legacy,
It is revealed that Five didn't meet the Mogadorians until at least the age of 13, right after 2 dies. This invalidates the question.