it would make sense, because you're supposed to milk cows more than once a day.

That depends on a number of factors, but if there are no current nursing babies, and if you are trying to maximize milk production, then yes, a 12 hour separation between milkings should be optimal.

But we make it easier on ourselves by milking at 8 AM and PM.

Various frequencies have been tried: http://wdmc.org/1997/MilkingFrequency.pdf

but technology has also advanced to the point where each cow can choose its own frequency: https://en.wikipedia.org/wiki/Automatic_milking#Advantages

I swear I read a paper once indicating you get a better milk yield if you do one milking in the middle of the night.