I took a look and these were on:
Snap To Markers
Snap To Ruler (coarse)
Snap To Frames
Switched them off and there's still a tiny little click at the join.
I would not auto heal it that won't remove the zero level.
I looked at your screen shot and from what I see there it looks like you are not zoomed in very far. But if that is an export of your joined files I have to say it just looks like you did not align things correctly. And I do not work the same as Fried CB. I do use snapping and I have it enabled to Snap to clips and snap to ruler(fine). This means that it can automatically line up on those marks without me having to worry about zooming in to get it exactly right. Then I can work with a file zoomed out far enough to see an hour or two yet get a perfect alignment.
Couple of questions....
Have you looked at your two files to see if that flat zero line is on the end of part one or the beginning of part two?
Two have you tried zooming your wave form in more and then aligning it and/or align it then zoom in and verify it is perfectly aligned before you do an export?
IS this your first time aligning files in Audition? If so then maybe you are not using the marker exaclty right causing that gap? It's been a while since I used AA3 but in ver4(CS5.5) as I drag the files together there is a white line that turns yellow as they align perfectly. BY having snapping enabled I just get it very close and then it kind of jumps(snaps) into perfect alignment. Hope some of this helps you out but I can assure you the software does perfect alignment even if you zoom down to the .0001 second readings