Positioning two opposite \fermata signs over the last bar line

To position two fermata signs on the last bar line (a typical setup in XVIII century music) add the Mark_engraver to the lower staff using \with.

\version "2.24.0"

%% http://lsr.di.unimi.it/LSR/Item?id=10

\paper {
  indent = 0\mm
}

\score {
  \new GrandStaff <<
    \context Staff = "one" {
      c'1 \bar "|."
      \override Score.TextMark.self-alignment-X = #CENTER
      \textEndMark \markup { \musicglyph "scripts.ufermata" }
    }
    \context Staff = "two" {
      c'1
      \tweak direction #DOWN
      \textEndMark \markup { \musicglyph "scripts.dfermata" }
    }
  >>
}