\version "2.21.0"

\partCombine
{
  c'1 |
  \partCombineChords
  \pitchedTrill
  a''2
  \startTrillSpan bes''
  a''2\stopTrillSpan |
  \partCombineAutomatic
  c'1 |
}
{
  c'1 |
  \pitchedTrill
  c'''2
  \startTrillSpan des'''
  c'''2\stopTrillSpan |
  c'1 |
}