How are you going to join the 2 battings?

I hadn't thought about the bats pulling apart if I used pieces of fusible on them -- good catch, Princess! I'm considering the bar tacks......

Interesting thought, Crafty14. I'm also thinking about a "modified" QAYG that joins up rows of blocks with the heavier quilting done before the backing was put on.

Yes, Lisa....good idea -- especially when combined with some of the other thoughts being offered.

A LOT of "food for thought" is being offered. Hmm! Meanwhile, last night I watched the free Craftsy Class called "Creative Quilt Backs" and got some more ideas. She does a LOT of careful planning using graph paper and there were many comments complaining about "all the math" but she gets terrific results.
Thank you ....and keep the ideas coming!

unless you plan to use this in a very cold climate, I think double batts would be too much. I would try a heavy flannel or a light weight fleece instead. You would still get some "puff" but not so much weight.

for the more simple side, you could just use a sheet to quilt it to.

I am not sure I understand completely...it seems you want to quilt the top and backing separately with batting on the back of each. And you are wondering how to stitch them together????
If this is the case...I would spray 505 to attach the two together then tack or sew however you decide.

Why not use flannel for batting; quilt each top, then spray glue the two together and consider some type of quilting that will join the two quilts; then bind.

I made a quilt like this several years ago. The front side was a circus tent and the back side animals and circus performers. I hand quilted them induividually using bridal tool as the backing and then joined them together with a big quilt stitch and bound as usual. I guess it was OK, since it got a second place at the state fair that year. If I could figure out how to post a pic on this board, I would.