  If $\mathcal{C}$ is a minor-closed class of matroids, the class
$\mathcal{C}'$ of integer polymatroids whose natural matroids are in
$\mathcal{C}$ is also minor closed, as is the class $\mathcal{C}'_k$ of
$k$-polymatroids in $\mathcal{C}'$. We find the excluded minors for
$\mathcal{C}'_2$ when $\mathcal{C}$ is (i) the class of binary matroids, (ii)
the class of matroids with no $M(K_4)$-minor, and, combining those, (iii) the
class of matroids whose connected components are cycle matroids of
series-parallel networks. In each case the class $\mathcal{C}$ has finitely
many excluded minors, but that is true of $\mathcal{C}'_2$ only in case (ii).
We also introduce the $k$-natural matroid, a variant of the natural matroid for
a $k$-polymatroid, and use it to prove that these classes of 2-polymatroids are
closed under 2-duality.
