Interesting, but could you think of a specific use case? I can see how conceptually it kind of matches Tuple, but I don't see a need for it.

I.e., I tend to think of Either<T1,T2> as combining two observables into one, where one side is the primary observable and the other side provides out-of-band notifications. How can we justify providing more than one out-of-band notification channel besides similarity to Tuple?

Perhaps people will vote it up if they need it. :)