LSR for two opposing fermatas fails on staff change

[Javier Ruiz-Alma]

The LSR for typesetting two opposing fermatas above/below a barline will not
produce desired result when voices converge on one staff.  Suggest adding a note
of this, or better yet, how one could overcome this.

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

\version "2.16.1"
soprano = {
        c''2 \change Staff = "lower" d''2
        \override Score.RehearsalMark #'break-visibility = 
#begin-of-line-invisible
        \mark \markup { \musicglyph #"scripts.ufermata" }
}
bass = {
        a'1
        \override Staff.RehearsalMark #'direction = #DOWN
        \mark \markup { \musicglyph #"scripts.dfermata" }
}
\score{
          \new GrandStaff <<
                  \new Staff = "upper" { \soprano }
                  \new Staff = "lower" \with { \consists "Mark_engraver" } { 
\bass }
          >>  \layout{ }
}