Well, that's just some kind of rubbing sex. If he penetrates one of them for real, then he obviously can't penetrate the other.
And if his cock's longer than 2-3 inches, he's going to go past the wet zone of the vulvas and his cock-head will be rubbing their lower belly regions. Which isn't bad (esp. with lube/oil!), but it ain't the wet velvet experience that is unmistakably vagina.