I always launder wide backing fabrics. Doesn’t matter if the top fabrics were laundered or not. Wide backing fabric tends to have serious set in wrinkles from being wrapped on the bolts- only way to get them out without hours of ironing is to wash it, toss it in the dryer, then take it out right away & shake it out- lay it out. I drape it across my longarm frame. I would launder both pieces separately then sew them together to avoid having a puckered seam line from uneven shrinking.