Joining two areas of tongue and groove flooring in a room - disguising the join
I've been thinking about this since yesterday. There are all kinds of transition pieces for floors that allow for expansion but they all have a slight overlap and thus are raised from the floor, like the one I've shown below.



If this is in the middle of a room it stands out like a sore thumb. I'm guessing you want a transition that's flush so if I was doing it, I'd route out the space like you said you wanted to do. I'd fit in a flush transition piece and deal with any expansion later if it should happen.