Comment 4 for bug 1510813

Harry, we allow both libs on the ppa builds to be co-installable, but the archive builds require a full transition anyway (all reverse deps get rebuilt). Also the debian guys claim there are issues having them co-installable, from the changelog

[ Andreas Henriksson ]
  * libgnome-desktop-3-12: Add Breaks: libgnome-desktop-3-10
    - Forces a full upgrade. This is to prevent applications linking to
      and using both at the same time (which leads to crashed). That
      happens in eg. gdm3 built against libgnome-desktop-3-10 when using
      gir1.2-gnomedesktop-3.0 3.18.0-1 (built against
      libgnome-desktop-3-12). (Closes: #801603)

Anyway the real solution to your problem is to disable xenial-proposed on your system, its not meant for human consumption.