I don’t seem to be able to find the URL of the document but the present plan is to develop the site in four phases. The first phase is to be the transit hub and the quadrant nearest Park Lawn and Lake Shore.
The planned GO station is indeed near Mimico Station. In fact, Metrolinx did not initially want to develop a station on this site for exactly that reason. Fortunately, they were overruled by City Council. The proposed station will link closely with other modes of transit while Mimico is somewhat isolated in that respect. And, obviously, the population of this area will be far denser than old Mimico.
It looks as if trains will alternate between stopping at the two stations at least initially. My own guess is that Park Lawn will quickly emerge as the main station in the area.
Edit: Here is a map of the phases of construction
https://urbantoronto.ca/forum/thread....19258/page-38
Scroll to the bottom of post 562