$ sudo aptitude install -y -f --no-gui --without-recommends
    -o Dpkg::Options::='--force-confnew'
    -o Debug::pkgProblemResolver=yes
    pkg_a=1.1 pkg_b=1.2

The following NEW packages will be installed:
  pkg_b{b}
0 packages upgraded, 1 newly installed, 0 to remove

The following packages have unmet dependencies:
 pkg_b : Depends: pkg_a (= 1.2) but 1.1 is installed.

The following actions will resolve these dependencies:

     Keep the following packages at their current version:
1)     pkg_b [Not Installed]

The following actions will resolve these dependencies:

     Install the following packages:
1)     pkg_b [1.2 (stable)]

     Upgrade the following packages:
2)     pkg_a [1.1 (now, stable) -> 1.2 (stable)]