The agent said it is because for a 2 stopover + turnaround trip, your turnaround city need to be in the middle. i.e. Stopover+Turnaround+StopOver. You are not allow to do Stopover+Stopover+Turnaround.
With Aeroplan reward I did:
YYZ - YUL - ZRH - FRA - SIN (turnaround) - HKG (stop) - KIX/ITM (stop) - NRT - ORD - YYZ
I don't believe there is a rule that says max. 1 stopover for each direction.