Yeah it's strange, wouldn't just putting up one arm with a 400 watt fixture make more sense? Unless they figured that having two fixture would mean if one went out the would act redundantly and light the road.
Apparently MTO did switch the lumes separately at one point since there's a article titled "Dual-level lighting system performance Highway #401, Oshawa" in their library...too bad they didn't scan the full text in though.
Anyways this setup looks late 70s - 80s to me.