As for combining two planes -- I'd recommend right clicking on you model, and left-clicking on "vertex" in the command-rose which pops up.

Select the move tool
Select the vertex (corner) you wish to move
Hold "V"
Grab the vertex and pull -- it will snap to the nearest vertex you drag it over.
After you're done, I'd recommend selecting each corner and clicking "merge verticies" from the "edit polygon" menu... that will increase performance in rendering/in a game.