The proponents of the alternative personality
theory in Pale Fire may be interested to read The Story of the Late
Mr. Elvesham (1897), something of a variation on the Jekyll-and-Hyde theme,
by H. G. Wells: http://www.online-literature.com/wellshg/2870/.
They will note the Kummel/Himmel pun (echoed by the Tokay/Bokay pun in PF?) in
it.
Alexey Sklyarenko