This was several years ago - so may not apply now -

A friend of mine wanted to sew two layers together - and they shrank at different rates - so it didn't lay nicely after

If you are going to layer two different ones, you might consider pre-shrinking before sewing together

so would flannel be better for a blanket back? i don't use batting.

Are you asking about using something on the back of a fleece blanket?

I've done the tie blankets and put two fleeces together and a friend has made them tied with just a single layer.

Normally I use fleece on the back of my quilts/blankets. I don't use batting.