On September 17 the official merger of the Archdiocese of Anchorage, Alaska with the neighboring suffragan See of Juneau, Alaska will take place creating the Archdiocese of Anchorage Juneau. The current bishop of Juneau, the Most Rev. Andrew Bellisario, CM, will be installed at Our Lady of Guadalupe Co-Cathedral in Anchorage and also will be invested with the pallium as a Metropolitan Archbishop. The sensible decision to combine the coats of arms of the two dioceses by simply borrowing elements from each was made. The new coat of arms combines all that had been in chief of the arms of the Diocese of Juneau with what had been in base for the arms of the Archdiocese of Anchorage. It’s a lot of blue as each one had a primarily blue field but the overall look is not unpleasant.
EXARANDORUM 
Yes, the combination of parts of the two coa’s of Anchorage and Juneau is a good choice. That ‘lot of blue’ is not hard. But why that partition line? The combination is one of PARTS of the former coats of arms not the complete one’s.. So the symbols can be combined in a undivided blue field, a real symbol of this unification.
I thought something similar. But, I can see that perhaps they wanted a way to illustrate that the combination creates a “double see” with two cathedral cities rather than one diocese merely being absorbed by the other.
According to the archdiocesan online newspaper called the Catholic Anchor, the partition line represents the horizon between the sky and earth. A description of the new coat of arms is given in the following article:
http://www.catholicanchor.org/featured/more-questions-and-answers-about-the-transition-to-the-new-archdiocese-of-anchorage-juneau/