The Companion Pass does not allow multi-city but the big difference is that I cannot book a return flight from CDG to YUL without going through YYZ using the companion pass.
Companion Pass = YUL-CDG, then ten days later CDG-YYZ-YUL, I would also need round trip YVR to YUL tickets to bracket the European trip and leave time in Montreal.
If I Use the Companion Pass from YVR I get YVR-YUL-CDG and on return I get the direct flight CDG-YUL-YVR (no YYZ)
It really doesn't make sense that I can have the direct flight from CDG to YUL in one case but not the other.
The price ends up about the same so no real financial gain or loss.