This is a small thing, but I think this sim should be as realistic as possible. To make sure I was right, I found a picture of a real Q400, and it does indeed have 2 bulbs in each light.
Maybe the devs thought it would be unnecessary to add both landing lights, as one set of landing lights is fully sufficient for Infinite Flight. Just my thought though.
Lights in IF are simply a floating ball of light that is attached to the plane. They’ll have to add another floating ball of light to simulate another light being there.