Often, with an old Chavrusashaft (partnership), certain patterns settle in. In many cases it ends up being one of them is always the one saying while the other will only speak up when he has something to say, or when he wants to repeat something to get it clear.
It is advisable, in my opinion at least, that even when you aren't the one reading out loud, you should be saying it to yourself. This avoids the issue of nodding along and fooling yourself until you regain consciousness. There is a benefit of saying the words of Torah. Chazal tell us that in that way you retain it. However, there are opinions that, as in other Mitzvos, your Chavrusa is Motzei you with the recital.