Thanks for the formulae, coldby
There's no way I know to combine multiple line surfaces --- you can get them into one object, even optimize out the dupe vertices, but the only way to convert them to a single line that I know of is a manual process:
1. select your two line segments
2. perform an Object -> Merge
3. ensure two of the vertices where the surfaces meet are in the same location (select them and use Vertex -> Snap Together)
4. perform an Object -> Optimize Vertices to get rid of the dupe vertex
4. select both lines and perform a Surface -> Remove Surface Only. This will leave your vertices intact, but will remove the lines.
5. in Vertex mode, select each vertex one at a time in the order you want them in your final line
6. perform a Vertex -> Create Ordered Surface
7. set the Surface Type to Line using the buttons in the bottom left of the AC3D interface
Would be nice to get a built-in utility to merge lines...