First, I would say the government or Parliament should answer this question, since the government decided to build the buildings in that location. Since the two agencies are side by side, it's easier for CSIS and CSE to meet and hold discussions. This is especially true since, according to the National Defence Act, CSE—which I monitor—can assist CSIS, under part (c) of its mandate. The two agencies are side by side, which facilitates discussions and meetings.
On March 21st, 2017. See this statement in context.