[aaminoff]

OK, I went and tested it.

First of all, if you script Stoneskin, Barkskin, the spell casting AI will look at you like you're an idiot and cast something else.

Using Strength of Gaia instead, it turns out that regardless of whether you cast SoG,Stoneskin or Stoneskin,SoG, you end up with the same effect: protection 15 and cold vulnerability. So it appears that the higher-protection skin always wins.