arpeggio across voices

[Jay Anderson]

Here's what I have right now:

%\set PianoStaff.connectArpeggios = ##t %this is close but not quite
<< {ees4( d8 ees4 des8)} \\ {<bes ges>4.\arpeggio <bes ges>\arpeggio} >>

I want these arepeggi to go from the e flat down to the g flat. It is
a piano piece, but the connectArpeggios didn't quite do it as I don't
want it to cross to the other staff. I tried faking it was hiding some
notes, but couldn't get it to work how I wanted. Any ideas?

-----Jay