When I try a quick test with two boxes the line between the two polygons is formed correctly.

So I wonder about the two polygons you input. Are the coordinates that both polygons should share really the same?

​@geomancer, this is the Above polygon…

This is the Below polygon…

If shown together you can see that they match along the shared edges…

Can you show us the result of your LineBuilder in the inspector with the geometry and the points it consists of in a screenshot? Like so:

I cannot be sure both polygons match along the shared edges. There could be small gaps and overlaps, there could be missing points, there could be differences of a millimeter between coordinates, there could be plenty of reasons why the edges of both polygons don't excacty match.

Here you go, ​@geomancer 

Can you show the information under Geometry in the Feature Information panel?

​@geomancer I’m confident that the polys do match along the main edge. I know this because the Below poly was created from the Above one. To do this I first created a doughnut polygon using the Above polygon (thus created a large poly with the area covered by Above as the hole in the middle). I then snipped it at the edges to create the Below polygon.  

I see. In that case, unfortunately I have no idea why your workflow does not produce a geometry, while my quick test does.

Maybe you will have better luck using a TopologyBuilder?

Can you test with a GeometryFilter that the feature is a line.

And then for the writer, what is the writer format? You may want to check to ensure the geometry aligns (not trying to insert a line feature to an area featuretype that already exists).