The "normal" process followed by pretty much all the spas (unless it's a special promotion) is to pay double room fee for Duos. So in your example, it would be $160 x 2. I confirmed this with a bunch of spas because I was also surprised and the response was you are paying 2 room fees because you are tying up 2 girls.
Now having said this, you can always negotiate, especially if it's a slow day. You probably have a better chance of negotiating at the Tier II spas as opposed to the very busy spas...