I'm not sure about that. As I understand it, W's argument against private language is essentially that there is no way for a single person to be sure that they

I'm not sure about that. As I understand it, W's argument against private language is essentially that there is no way for a single person to be sure that they are following rules and that those rules are consistent over time. It also happens that none of the languages we are familiar with use thought as their medium (being signed, spoken, read, etc. instead), but if these twins are indeed using internal dialog (whatever that means) as a medium of communication, it could very well be regulated by rule following in a way that would satisfy W, don't you think?