When I've made lined curtains, I've only attached the lining at the top and sides. Otherwise it never wants to drape right. Just hem the bottom of both before sewing together, and make the lining about 1/2 to 1 inch shorter than the main curtain so it won't show.
I like making curtains for the type of rod that uses clips on rings, so I don't have to make a rod pocket or tabs - so simple and quick to make. But other than figuring out the hanging method, I think it probably IS just as easy as your mind is telling you it is.
