If I was doing it, I would make the lining the full length. That is, unless using both layers makes the drapes too stiff to puddle.

Thanks Sally, I'd begun to feel I was overthinking this but your drawstring-at-the bottom makes a lot of sense & the degree of 'puddle' is ideal. Even though it's only 2 windows it's a lot of money in fabric & I want them to hang perfectly.

The situation I'm dealing with is a historic house with large (& drafty) windows & architechurally significant woodwork where the treatment needs to be elegant but non-distracting & can help slow air infiltration in the winter. The windows are very deep & not quite as close to the floor as the ones above (& if that's your work, it's fabulous) so I'll be be doing an inside mount with the puddle on the sill, probably in a lined & interlined velvet. Hopefully, the trim will remain the star & the treatment will add depth & prevent the breezes when the curtains are closed.

If you are opening and closing your panels, I would just have them "break" on the sil. It will look much better and help with the insulation just the same. Interlining is good, more layers make for better insulation. What type of rod will you use as an inside mount for the panels? They will be heavy with velvet and interlining.
The panels in the pic are interlined, but I must give credit to our large workroom for making them----they were too long for my little workroom---but I had them leave the bottoms unfinished so I could put in the drawstring hem. I did make the overlays and they are buttoned to the outside pleats.

In a perfect world, both of us would prefer that they break on the sill, but the sill seems to be the most prone to air leaks & thus the purpose of the puddle. Most of the window will be covered when the panels are open so they will be pulled together only in the most extreme temperatures. My sister is aware this is likely to be a PITA & I seriously doubt they will be fully closed more than once or twice!

An inside mount rod has been a challenge & I'm going inventive. (Like the draperies, I'll be doing a mockup before hand to see if this actually works.) For the rod, I plan to use a heavy duty copper plumbing pipe (easily obtained & cut to exact length & can be reinforced if necessary) mounted to the upper stop with a type of flat stock pipe hanger that's open on one side (forget what they call it but you can get it at any box store in copper or steel). I'm becoming an expert in copper plumbing hardware much to the amusement of the salesmen! The pipe will lay in the hanger similar to a regular outside mounted pole except the hanger will be attached to the underside of the jamb instead of the wall. To solve the problem of the rod-pocket-on-a-hanger, my anticipated solution is to put 2 buttonhole type openings on the back of the rod pocket lined up with the hanger so the rod can be woven out of the pocket & exposed on the back for the hanger without being seen from the front. I hope this makes sense because it reads horribly.

Your comments & advice are much appreciated; most everything I've found on the internet consists of only the most basic solutions & this is definitely NOT that kind of project. I tend to be a problem solver & that's what's gotten me into this ordeal - much less on a volunteer basis.

I don't get to live in that "perfect world" all the time, but it's fun when I do! Certainly not for myself, I guess that's why I started making window treatments in the first place. So congrats for thinking this through so well.

It sounds like you've figured out how to cover the bracket. Too bad there isn't some socket mount bracket to fit the tubing. Since you are not using rings and finials, the cost of a rod/socket bracket wouldn't be too bad if you have a local home decor shop to check out. It's the rings and finials that really drive up the price. I'm not picturing the attachment/rod pocket exactly, but I know you have it all in you head!
I do know some of my window treatment friends do roman shades for older homes that are interlined, and even quilted, to keep the draft out. That, with panels on the sides, is really a good way to insulate those old windows.
I guess you've figured out how to cover the gap between the pole/rod and the curtain with a header above the pocket? I hate seeing daylight on inside mounts above the pole, more from the outside than the inside. But there are little things that bother me and not others.
I hope you'll post your finished results, I think we can all learn a thing or two with these "not perfect" situations!

Offhand, I don't remember why I discounted the wooden pole & socket assembly but I know I considered it; it may have had something to do with the wide angle of the window jamb. Never fear - a header is planned & there will NOT be light showing around the edges - I can't stand that either!

Thanks again. And if this actually works out, I'll be sure to post.