Nicholas and Nathaniel normally don't keep two separate conversation at the same time. They have nothing against doing so, it just tends to not come up. When they do strike up separate conversations, they look a little weird. The brothers think it's hilarious, when they're shown photographs of it. Them both having to stop talking at the same time to take a breath isn't the issue. It's just how they look with their heads pointed in different directions and gesturing with their hands to separate people. It has the same sort of feel as someone with their eyes pointed in different directions. Only people who aren't part of the conversation tend to notice it though, so it doesn't really impact their interactions with others.

This super cute KWIK-I sketch was done by the talented  larru-larru. Find the original here, https://www.furaffinity.net/view/57210255/