With the prewashing and drying, it's unlikely either muslin will shrink a lot in the second washing. Some shrinkage in the fabric is okay anyway; it just adds to the crinkle quality of the quilt. Given the fact that you have prewashed and dried the fabric in the dryer (most of the shrinkage comes in the dryer, because of the heat), I think you will be fine.
Also, quilting itself controls shrinkage. If you want to be extra safe, just quilt more closely. I did a class with Harriet Hargrave once and she showed us a flannel quilt she made to prove to her students that prewashing isn't necessary, even with flannels that ordinarily shrink a lot. She used unwashed flannel, quilted fairly heavily, measured the quilt, washed and dried it, and measured it again. Those fabrics would have shrunk considerably if washed by themselves, but did not shrink at all in the finished quilt -- because the quilting stabilized them. Not sure I'd do that myself, but she did prove her point.....