[Wynton]

I can't say anything about the extenders, but have you considered a second tablet (doesn't need to be a second surface, a cheaper tablet that's big enough for you will do) in book mode?

Not recently.  I've been assuming that a second tablet - of an appropriate size - would be more expensive than a second screen, like one of the extenders.  Also for me, I might have a work need for an extender anyway.