In message , at 13:36:35 on Sat, 19 Dec
2009, Sir Benjamin Nunn remarked:
Couldn't they have put the Jubilee at a direct right angle to the
Northern Line, at a slightly lower level and with an equally generous
'island' between the platforms, then just had a bank of escalators from
platform level to platform level?

As far as I know this is pretty much what happened. The only reason for
not making a 'direct' connection by a short flight of stairs (between
the two sets of platforms) must have been because it would have been
"too popular" and they wanted to create large circulating areas
connecting mainly to the street, instead.
--
Roland Perry