Joining 2 halves of a wall
Build the walls with a double top and double bottom plates. Span the splice between the sections with as much length of the plates as you can.

In my jurisdiction, wood in contact with cement has to be PT, so I'd use that for the bottom plate. 

Get the bottom plate nailed/screwed/bolted down first, raise the wall sections onto it, brace those wall sections temporarily. Then bang your extra top plate into place and nail or screw it to the wall sections. Attach your top plate appropriately to whatever's above it.

ps, watch the height of the wall across its length... garage floors usually slope and ceilings are usually level...