I like to use lacemakers' pins, which are longer and finer than regular pins, so they cause less "jump" if I leave them in and sew over them.
As for the chiffon overlay, it all depends on what effect you are going after. The more layers of chiffon, the less you can see of the satin underneath. If you want flowers they can always be sewn to the inside of a single chiffon layer. There are also some gorgeous embroidered chiffons out there, or crystal chiffon etc.
In any case, be sure your seams are as perfect as you can make them because any unevennesses will show. If you have a serger serge the seams, or else make the seams very narrow and then use a small close zigzag stitch to join the raw edges of both seams together.
To finish the hem, you can sew a narrow hem, edge with lace, or use a tiny close zigzag stitch to enclose a length of nylon fishing line to give that slightly stiff narrow edge. If you pull the line a little to gather as you sew it will ruffle as much or as little as you like.
Good luck!

If you have a serger you can make a rolled hem on the chiffon. I would make this layer the same as the skirt layer and the baste them together at the top before making the gathering stitches. That way it will not be too bulky. Test out your stitches on the Satiny fabric and if you find it too slippery you might want to cut strips of interfacing to sew along the seam line. You can use the tear away stuff. Colleen

Thanks for the info Colleen. I don't have a serger...I don't even want to tell you how old my Bernina 'workhorse' is, but it works.
I like your idea of perhaps sewing flowers to the underside of the chiffon.
I'm can't visualize how you said to finish the hem. If I understand you correctly, if I zig-zag over a length of nylon fishing line, and then cut the fabric on the outside edge of the zigzag, my hem will have a finished look?
Thanks for the tip on the interfacing.
Would the hemline look better if the chiffon overskirt and dress were hemmed separately? How would it look if they hemmed together? What about the doubling the chiffon?
The problem is, the flower girls are sisters. One is too young...16 months, so her older sister (9 years old) has to help to make sure she walks down the aisle (It's an informal wedding). So the finished look of the dresses has to be appropriate to the older sister also.

If you use a narrow, close zigzag stitch on the fabric edge, encasing the fishing line as you sew, it will be finished when you get all way around. Don't attempt to cut a piece of fishing line. Sew right off the reel/spool. I wouldn't hem the satin & chiffon together. The chiffon needs to float. If using tulle, I would consider doubling it but I wouldn't double chiffon.

Bubble skirts are popular now out of satin. It has an underlining that is fairly straight & shorter by a few inches & the bottom of the top layer is gathered to it. Both attach at the waist in the same seam. There is no pressing to flatten the affect so there is no exact hemline. Most of these are knee to tea length.

As far as dress styles goes, the two don't have to be the same. No-one would expect a 16 month old to have the same style dress as a 9 year old, just as they wouldn't expect a 9 year old and an 18 year old to be identically dressed.
There will be enough "match", if matching is important, if you use the same materials in both dresses.

Yes, do the chiffon hem separately to the satin hem. Apart from that it will look better, you will have a hard row to hoe if you try to hem them together AND have both layers hang properly together.

Joan has described the fishing line technique a little more clearly than I originally did- you lay the fishing line right on the edge of the chiffon (or a poofteenth in from the edge).

I don't understand why you seem to be set on the idea of doubling the chiffon (you've mentioned it in both your posts now). As I said in my earlier post, more layers of chiffon will obscure the underneath fabric. So if you double the chiffon, the effect will be to make the dress look like it is made of chiffon, rather than chiffon over satin (as it would look with one layer of chiffon). In that case, you may as well save the expense of the satin and use something else for the underdress so as to feature the chiffon. (And if you do double the chiffon, hem each layer separately.)

As the other Colleen mentioned, you should make the satin skirt and the chiffon skirt sections separately before slipping the satin skirt inside the chiffon skirt and then basting them together at the top and gathering as a unit. Then attach, treating the chiffon side as the "right side of the fabric" for the purposes of the pattern instructions. By doing this the side and back seams of each layer will be separate from each other- they should not be sewn together, except at the zipper, if there is one and it extends that far down. I would wait to hem until the dress is completed and the hem has had a chance to do any dropping it is going to.

Thanks for the tips, Colleen. I knew I would want to sew both chiffon and satin separately, but didn't think about the zipper. Good point, to sew that part together.
The reason I mentioned "doubling" so many times is that my daughter (the girls' mother) had seen that on a website for flower girls dresses, but I didn't think it looked that great.
You're right...I would want the satin to be visible.
Talked to our contractor..and I do have several more weeks, so
I am not quite in the panic mode yet!
BTW...where can I find lacemakers pins?

Well, I'm in Australia but I get them at the sewing notions shop. They shouldn't be too hard to find.

make sure your sewing needle is sharp, and change it out every couple of hrs, if not more. if you have a dull needle while sewing the silk, you will pull the threads.

I know it's been months since I asked for help, but I wanted to show you the two dresses. I was a lot more confident making the dresses because of the helpful tips I received. The wedding was the end of April...I had to finish the dresses in very adverse conditions (cold and cramped quarters) because our remodel was in full swing.

Those turned out beautiful! I'm hoping to make something similar for the GDs for the holidays. Your pics give me a bit more confidence. Gorgeous flower girls!

The dresses look lovely, as do your little flower girls, Ashlander! I'm glad you posted these photos- thanks!

What a nice job you did on the dresses! The little girls look lovely! And thanks for returning to let us know how everything turned out.

colleenoz:
I'm so glad you saw the pictures. I followed your advice on the hems. I experimented with the fishing line, but didn't like the look.
I actually used two different hems on the dresses. For the older granddaughter I used the hemmer foot and rolled the hem because her skirt was more of a princess style, gored. (I think that is the term)...not a lot of gathers.

For my younger granddaughter whose skirt was gathered, I doubled the hem and hand stitched it.
Thank you all for your compliments.
I'll be "back" on this forum periodically, as I undertake other projects. Right now we are trying to unpack the rest of our boxes, following our remodel.