More than 12 zones

Before I really knew what I was doing, I ordered an Alarm Panel Pro kit that came with two interface boards. I quickly realized that the interface boards were not what I needed or wanted.  I then ordered two 8 zone addon boards as I was told "this is what you want".  Well I'm not so sure.  I had been under the assumption that the addon boards would tie into the main alarm panel using the two expansion headers.  But I read somewhere on this site that's not how it works and there isn't any wiring connecting the addon boards to the main panel.  This doesn't make sense to me.  If that's the case then what I would essentially have is three alarm panels.  If anybody would like to straighten me out on this, I'm all ears.  But from where I am right now, it would appear as though I should simply have two Alarm Panel Pro boards in order to get past 12 zones.