Thank you ladies for all the suggestions! The old blanket that I am using on the back of my bargello, is really, really thick. I am not using batting because it is so thick. I tried last night to quilt it after I turned it...it was a no go. I didnt get even half way in on row before I started having problems, because the blanket for the back is so thick. So I ripped the stitching out and have decided to just tie it.
What I did...I placed the top quilt face down, on top of the blanket faced up. Pinned ever 3" or so and then stitched around the edges, then flipped/turned the blanket right side out. Now I have repinned ever 3" again. It is actually very flat, no puckering. I now need to get some embroidery floss to tie it. I tell ya tho, it is HEAVY hahahaha The blanket that I used for the back is one I had when I was a kid and it was just in the closet all these years. And I thought, heck, I can reuse it and just put a new face on it. My husband really likes it. I have found some other blankets in the closet that I am going to ues for future backings ;)
Once I get it tied, I will surely post a picture and show all of you :)