Seam finishes when overlaying with tulle

I'm making a garden party in slinky lilac/blue synthetic fabric, which I have overlaid with a nylon lilac/blue tulle. I have cut the tulle longer than the dress. I have sewn the dress by treating the tulle and fabric as one throughout. Due to the tricky nature of the fabric, which frays A LOT, I was planning to do french seams. I'm close to sewing he front to the back, and I'm now wondering how to do the side seams. If I french seam them, then the tulle which hangs lower than the fabric will be all bunched at the side seams, if that makes sense. In any event I'm not sure how to sew two layers of tulle together. Do you have any advice or suggestions please! Thank you.