First thought - Why do it? Why not just buy 2 rail ones to begin with.
Second thought, it really is almost that easy - - depending. If you don't need electrical pickup for lights or something, changing the wheelsets AND removing the center rail contact is all that is necessary. You also have to figure out how to mount couplers My experience with converting a few 3-rail freight cars that I picked really cheap is that Atlas use the same under frame for 2 and 3 rail, so the mounts may be there - drilled and tapped for 2 MM screws. However they may to be shimmed lower.
If you want electrical contacts, then it becomes a little more complex. The simple and quick way is to just replace the trucks with Atlas 2 rail passenger trucks that already have contacts. The much cheaper solution is to design and install your own with phosper bronze wire or strip. Any springy wire or strip should also work.