In typical Metrolinx fashion, they are redoing something that they had previously and recently undone.
The original connections at Canpa (the railway point where the lines connect) allowed for two paralleling movements. When Metrolinx purchased the Canpa Sub from CP, they "rationalized" it to a single connection, in order to save on maintenance and to make the resignalling of the interlocking easier when it came time to exend the fourth mainline track from Mimico.
Of course, this "rationalization" caused many delays to the trains during the Lakeshore West detours of 2 years ago (it was that long ago?!?). And so in the post-work wrap-ups, they identified a need for a second connection. I guess they've finally found the time to do it now.
Dan