They are a little bit different.
the phrase construction 皮+膚 = skin + skin -> it indicates only the surface -> superficial
while 肌 + 膚 = muscle + skin -> it's more than superficial -> figuratively means intamate relationship.
When you try to compliment your girlfriend who has a flawless skin, you'd better say 妳有光滑無瑕的肌膚. And if it is at the right time and right place, maybe both of you'd like to have a 肌膚之親。