vertical shift of trill

[Jim Cline]

Hello, I have a passage with a double trill.  I think it looks bad to have 
the lower trill symbol so far from the note, and would like to have it 
closer, overlapping with the staff.  Could not find any documentation on 
this so far.  regards, Jim

\version "2.20.0"

\relative c'' {
 \clef treble
 <<{ \pitchedTrill
  c'1~\startTrillSpan des c1*7/8 s8 \stopTrillSpan }\\{\pitchedTrill
  a1~\startTrillSpan bes a1*7/8 s8 \stopTrillSpan}>>
}