What size are you making Rebecca? From memory I think the flounce for the tunic ans the flounce for the dress are separate pieces? It has been a while since I had contact with that pattern but I think I was able to cut it with two separate pieces of pattern. Anyway forgive my rambling, if they are the same and you want to do a layered effect without adding an additional piece to the tunic in order for the flounce on the hem to sit lower, you could draft a lengthened version of the flounce pieces and have this as your bottom flounce. If you want that effect but don’t want to sew the two flounces on at the same point then you will need to figure out the amount you will need for an insert that will be attached to the tunic at the first flounce and then attach the lower flounce to that insert.
If you were planning the route then it will be important to pay attention to the fact that the insert piece needs to also look the same with the curvy part at the front to accommodate the flounce.
I sincerely hope this makes sense and is helpful – sitting for five minutes having a cuppa in between an extremely busy non sewing weekend, so forgive me if I don’t an ounce of sense.