problem with line breaking with polyrhythm

[Libero Mureddu]

Hi list,
I'm having problems trying to write scores for some pieces with polyrhythm:
lilypond says that:

warning: can't find line breaking that satisfies constraints

I put here a minimal example that shows anyway the problem, my real situation is on a much bigger scale, that's why I need to force in that way the notation, and I have to split a measure in the middle.

In many examples in the documentation I've found tuplets splitted in the middle, but when it comes a polyphonic situation (the other voice doing something different), the same warning appears.

Any suggestion or workaround?

Best regards,

Libero

%%% SNIPPET FOLLOWS %%%
\version "2.8.6"
    \score { <<
        \new Staff {            
                    \time 4/4
                    a4 a4 \bar "" \break a4 a4
                      }

        \new Staff {
                       \times 4/5 {
                    a4 a4 a4 a4 a4
                    }
                    }
            >>
        }
\layout {
  ragged-right = ##t
        }