Why not try to build in a feature like a raised path, a flower bed/border, a marked pathway of a different colour tarmac. Go have a look at a real carpark and see what they add.
You should be able to get a good butt joint, bit if you cant get it as good as you can, and add a black line along the joint to represent a join in the tarmac. A real joint is covered with a fine strip of tar just to cover the join and also add a bit of glue to the edges.