I'm able to see during the rising and falling of my breath at my stomach that there are two primary phenomena. One is the tactile feeling of pressure in my abdomen as the breath enters or release as the breath exits, and the other is a vague mental image of the abdomen rising and falling. I associate this second phenomenon with "mind" and the first with "body." 

But although I can tell fleetingly that these two phenomena are distinct, they are for the most part mixed up together, and I have trouble consistently seeing them as separate phenomena when I am paying attention to my breathing. I try often to notice exactly when I am feeling a tactile sensation and when I am experiencing the mental image of the breathing process, but I find this very difficult and can rarely see the beginning and end of either process, even though I've been at this stage for hours at this point.

I have two questions. 1) is this the right thing to be looking at when I'm meditating? I suspect that these two phenomena are two sides of the mind-body split if the first stage of insight (1st Nana), but is that right? 2) if the answer to  the  previous question is yes, is there any advice y'all could give me in how to more quickly advance to a stage in which I can see the beginning and end of these two phenomena more often and more distinctly? Like I've said, as things stand, I can tell that tactile sensations are different things from mental images of those phenomena, but I can't in practice consistently see or intuit a sharp line between them.

I’d just work with the technique (noting or mindfulness of breathing) and let the details sort themselves out. If you’re getting the general idea of it most of the time, you’ll start moving through the stages soon enough. Don’t bog yourself down. 

Hi Ian,

Just to offer an alternate perspective, you say:
It's good that you are seeing the two aspects (tactile sensations and mental image) of the breath as being distinct.  Is it possible that the two aspects appearing as they are, being mixed together, is the way the breath is?  You seem to want to see them as separate phenomena but is it possible for you to have a mental image of a breath apart from the tactile sensation or the tactile sensation apart from the mental image to percieve the phenomena as a breath?  Is there really a "sharp line" between them as you think there should be?

It may be interesting to keep this question "lightly" in mind without going into discursive thinking, doubt or holding an expectation in your mind as to how the experience should be.  As you do this continuously, you may start to see what you previously thought of as a solid sense of "breath" being transformed into vibration of sensations or you may see there a kind of vacuousnes, spaciousness or emptiness in place or around the breath, which I think is an indication that your investigation is working.

Once you become skilled in this, you may wish to turn the light of investigation around, so to speak, to investigate the subjective sense of "self" or "I" instead of objects like the "breath".  What are the components that make up the sense of self?  I've briefly touched on this kind of investigation in this post.

Hope it helps.