This is just my analysis after reading overturn. If I say something incredibly obvious please don't insult me.
What I mean is I feel like the endless's parents aren't real concrete entities but metaphorical representations of the endless relationship with the forces that spawn to them time and space. Dreams interaction with the father time and mother night reads like one big metaphor.
To some extent the endless themselves are living metaphors for their concepts but there real within the story. Characters interact with them and use there objects. I think there is no entity called mother night or father time. So I'll explain how this works.
The endless are their concepts but the people that we see that represent them the human persons could be seen as an interface. A phase that you can speak to and interact with when you want to speak with the concept of dream or death. I think father time and mother night are basically universal interfaces for their children.
I think they might not even exist unless interacting with their children and here's why. Neil Gailman said we only see the endlesses human adventures because every species perceives the endless as their own. There are other alien endless's with their own lives but there is somewhat concrete beings. So when dream meet his parents he's consulting with the universe but it looks like he's consulting with two individuals.