I have never done it but see no reason why you can't. I guess the question is whether you would rather have a long, 6'x16' GH or two greenhouses, back to back. With joined GH's, you could have doors at opposite ends which would have advantages. With a back-to-back layout, you could build another one practically any size you wish. You would just have one door facing one way and the other door facing another way unless you put them side-to-side rather than back-to-back.
