I have had two variations of this occur in cultivation. One was as you
descriped on a green pitchered form of N. sanguinea. The midrib divided about
halfway up the leaf and two pitchers of slightly dissimilar size were
produced. The other was N. veitchii which did produce a Siamese trap, one
tendril at the end of which were two pitchers joined along their entire
length, the lids free and seperate.
Other strange events include N. veitchii producing a crestate inflorescence,
and N. rafflesiana producing one gigantic male flower with multiple sepals
instead of a regular inflorescence.
There is also photo I have seen of the twin pitchers from one leaf of a
whitish form of N. rafflesiana.
This would be very interesting if it could be a fixed trait and occur on each
leaf!
Cliff