I have made a few two layer quilts with fleece under pieced cotton, and I like the way they drape. They're lighter weight than a regular quilt but pretty warm for snuggling on the couch. I think any style of quilting or tying would work well. In reading this thread I began to think about possibly just quilting together a couple of pieces of fleece some time. I had not considered doing that before, but it would be an interesting candidate for a whole cloth project. Machine stitching shows up well on the fleece. Anyway, I'm just "thinking out loud".
I'm pretty sure that the piles and piles of fleece throw kits that turn up in Jo-ann's every fall are meant to be fringed and tied together on the edges only. There are youtube videos demonstrating this technique. It seems to be a popular concept because they have sold the kits for a number of years and always stock them again. I don't think there's a big problem about the fleece layers slipping against each other because someone gave us a commercially made fleece throw one time that was 2 layers with a machine blanket stitch around it. It was quite some time before I even realized it was two layers. Actually, I think it wasn't until after the kitty pulled out some of those stitches. I would say you'd be just fine with the project as you'd originally planned it. If the possibility of the layers slipping worries you, you could put a big X or + from corner to corner and/or side to side using a running stitch, maybe in a contrasting hand embroidery thread so it's a feature. Any way you do it, the grandkids will know you were thinking of them.