I don’t know about others but I don’t use my tongue. I can block my nasal passage and breathe through my mouth. I can have my mouth wide open and do this. It must be controlled by some muscle but I don’t know what.
I am pretty sure I am breathing through both right now (trying on purpose to do so). I can definitely feel the breath on my tongue and hold something to my nose and smell it.
Whether air from both places actually goes into my lungs? That, I don't know. So you may be right in that regard? I really don't know. :)