How do I merge two walls in archicad?

Select both walls, go the to Model Menu>Select Complex Profiles>Select Merge Walls. While it is perhaps not the most obvious place to look, it does work and your previously split walls will become one.

How do I join elements in archicad?

Use Design > Connect > Merge Elements. You are prompted to choose the element you want to merge it with.

What is junction order in ArchiCAD?

If two or more Walls meet (their reference lines intersect), use Junction Order to control the element level junction. Changing Junction Order determines which two Walls meet first, before others join the intersection. Set Junction Order in the Model Panel or Info Box of Wall Settings.