Hey thanks everyone!! I think I'll just use the 2 different kinds & spread it out, unless it ends up being way to hard to quilt through. And, the more I look at it, the thicker muslin is slightly darker too.. must be the difference between sunlight & fluorescent.
I didn't even think about it not actually being muslin! I had trouble finding the grain line (well, without the selvage, I wouldn't have been able to), it seems like both directions of the material stretch the same amount. Is that how muslin normally is?