Thankyou! All along I had been trying to change the wall finish of the left hand wall from the 'materials' tab of the 'Wall Specification' dialogue box and for some reason that was causing the two adjacent walls to merge together again after I had already split them apart. I watched the video and was about to define a new wall type as you suggested when I (finally) noticed the 'wall covering' tab just below the 'wall types' tab. I was able to apply the shiplap there and as you can see it worked! The left side of my wall behind the mirror now has shiplap and the right side behind the shower is still painted gray like I wanted without merging the two sections together. I'm still going to define a new wall type but this works for now.