Breath through your nose and mouth at the same time

I think most athletes breath through both at the same time, and it's not necessarily a conscious thing, it's just how you maximize your air intake. There's a funny story about Chris Froome doing a TT at the TDF one year and feeling like he was going anaerobic partway through despite putting out his usual power numbers, and then realizing after the race that he had left his nostril plugs in (he soaks plugs in some sort of oil to open up the airway but takes them out before the run).

I haven't paid much attention to it lately (similar to the cadence post either hehe) but it seems I do both as well. When I'm focusing on controlling my breath I believe I use my mouth more. Are you having any issues breathing or just wanting to maximize oxygen intake?

Air and water are going to take the path least resistance. I'm sure you breath a tiny amount from your nose without you realizing it. It wouldn't really make sense to try to breath less from your mouth to try to breath from your nose more. Your mouth is bigger and thus it's easier to inhale and exhale through it.