First, a plea from the heart.  DON'T DO IT!  A 65/88 pedal Steinway
grand is a rare and wonderful instrument.  Many tens of times rarer
than the Duo-Art version.  A conversion's never as nice as an original.
You'll be able to find a nice original Duo-Art quite easily.  There are
pedal-electric models to be had (common in the UK, although very
uncommon in the USA).  As was said yesterday, they aren't intended to
pedal DAs, only 88-note rolls.

Technically it's possible to have Duo-Art and 65-n holes on the same
tracker bar, because I saw one last year (I didn't look to see whether
original or a conversion).  I know from someone who converted a 65/88
pushup to Duo-Art that there's a lot of technical details such as
altering the split point in the stack, splitting the primary valve
rail, etc.  Keeping 65/88n pedal operation would involve additional
work such as extending the 65-88 crossover (a milled block switching
mechanically) to cover the Duo-Art tubes, fitting a 6-pouch
pedal-electric Duo-Art box and a second set of theme primaries.

Original Aeolian conversions of these instruments would have replaced
the double-valve Pianola stack with a single-valve Duo-Art stack.  
They did not retro-fit a hammer lift so these instruments aren't full
Duo-Arts anyway.  The huge early double-valve stacks do work with a
Duo-Art box, but Aeolian for whatever reason never shipped double-valve
Duo-Arts, so there must be a performance question about making such a
conversion.

Julian Dyer