Dummy knows more than declarer
From Law 70.B:

B. Concession Defined

Any statement to the effect that a contestant will
lose a specific number of tricks is a concession
of those tricks; a claim of some number of tricks
is a concession of the remainder, if any. A player
concedes all the remaining tricks when he abandons
his hand.


Declarer's statement states that regardless of the play, he will lose both tricks. This meets the definition of a concession. Your opponents should have accepted the concession and ended the hand without further play.
As far as what the dummy should do, the best option would be to ask &quot;is that a concession?&quot; as it provides no additional information. Regardless of what you do at this point, the hand should be over.