Curtains lined with the same material: where should I put the seam?
If you will mostly view the curtain from one side, handle that as the 'side you look at in normal curtains', do have that side a bit bigger and fold the edges over to seem them.
On the other hand, if you will see the whole of the curtain from both sides and you will want both to look nice, two pieces of fabric the same size, put the good side on the good side and sew around the outside, I would leave either the top or the bottom open for now. Turn and top stitch just inside the edge.
Most likely you will do something in the top seam to hang the curtain. If you have the skills to do that with the two layers of fabric finishing it nicely at the same time, keep the top open.
Otherwise, close the top and keep the bottom open and 'only' top-stitch that closed. (You can even seam both sides individually and leave them open at the bottom, your call, if will fall better but is more likely to get out of shape.)
If the side seam is shorter or more out of sight you can use that as the last instead. You can play around quite a lot, as nobody but you will judge your quality of work.