  We show that the simplicial volume is superadditive with respect to gluings
along certain submanifolds of the boundary. Our criterion applies to boundary
connected sums and 1-handle attachments. Moreover, we generalize a well-known
additivity result in the case of aspherical manifolds. Our arguments are based
on new results about relative bounded cohomology and pairs of multicomplexes,
which are of independent interest.
