[quote=DrowsyPangolin]But how do we know the other Walden is the real one?[/quote]
Because I believe earlier we read about where he went at the time of the bell of the Thistle.
And, if it were Sir Walden, he'd be in the company of the other 3 knights, not just alone. I doubt they'd let him disobey Thomas's orders, and it doesn't seem like they've been carried out yet.