How do I join walls in archicad?

How do I join 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.