Pairs of paths with the same source and target but with no other nodes in common are called *parallel paths*, at least on the computer science side of things in graph theory -- you can google the term to get some relevant CS papers, but I couldn't find a wikipedia article on parallel paths. 

If your paths are allowed to share nodes other than the source and target, I would suggest something like "piecewise parallel paths". With this terminology you can simultaneously get the idea across and score cheap points for alliteration.