My Ddog would certainly prefer two shorter walks, ideally different ones. He poos twice a day and definitely prefers to do that on a walk. He snuffles around on his walks and his nose almost seems to get tired on a long walk - not as daft as it sounds as smells are stimulating for them
I think both are fine, it depends on the dog and what suits you. My 2 large dogs get one long walk, mostly off lead. I don't want to have to clear up muddy wet dogs more than once a day. Sometimes one of them gets a lead walk later if I'm going somewhere. If they've got any excess energy they play fight in the garden but mostly they just mooch and sleep.