doubleSlurs in single-staff polyphony

[Daniel Rosen]

I'm trying to typeset a passage with two voices on the same staff. Both voices 
use double slurs, and I need the slurs within each voice to be parallel (i.e. 
both slurs in the top voice need to curve up). What would be the best way to do 
this?

\version "2.16.1"
\relative c'' {
  <<
    { \set doubleSlurs = ##t <b e>2( <c f>) }
    \\
    { \set doubleSlurs = ##t <b, e>2( <c f>) }
  >>
}

DR