Finally got a useful useful response from one of the Toronto area coin and banknote dealers that I contacted directly. Thought I would share it with you.
The broker replied, "consecutive pairs for the 1923 shinplasters are unusual but not unheard of. I happen to have a pair in stock right now in fact. In general, however, I would say that it is genuinely rare to see consecutive pairs of any ("any" was underlined) note dated prior to 1937 and even consecutives in the 1937 series are scarce."
He went on in a second email to describe a pair of 1923 McCavour / Saunders shinplasters that he has, both in AU condition, which he is prepared to sell for $375, $75 above the Charlton value for the total of the individual notes.
Hope this is useful info to one or two other collectors out there.