Well #1 you could always request your complete resignation from the HQ of your former organization... If they don't grant it, pledge somewhere else and force them to
#2, colony members are typically not considered full members. In Sigma Nu I was the equivelant of a candidate (pledge) for the 2 years that I was a colony member (now I'm a founding father and I like that better).
Honestly, I don't think it'd be double membership. That only refers to a FULL membership as far as I know.