  The merging of succinct data structures is a well established technique for
the space efficient construction of large succinct indexes. In the first part
of the paper we propose a new algorithm for merging succinct representations of
de Bruijn graphs. Our algorithm has the same asymptotic cost of the state of
the art algorithm for the same problem but it uses less than half of its
working space. A novel important feature of our algorithm, not found in any of
the existing tools, is that it can compute the Variable Order succinct
representation of the union graph within the same asymptotic time/space bounds.
In the second part of the paper we consider the more general problem of merging
succinct representations of Wheeler graphs, a recently introduced graph family
which includes as special cases de Bruijn graphs and many other known succinct
indexes based on the BWT or one of its variants. We show that Wheeler graphs
merging is in general a much more difficult problem, and we provide a space
efficient algorithm for the slightly simplified problem of determining whether
the union graph has an ordering that satisfies the Wheeler conditions.
