Merging is a bit of a misnomer.
You can open two models in two different instances of the program and use Copy to copy a line and Paste into another. It is one line at a time.
Renumbering Hydrographs can be done with the same methods - cut and pasting hydrograph rows.
On the Hydrographs tab, you can move rows down by selecting a row and inserting. This shifts the hydrographs down a position.
Delete will remove the line. I think it only works with Runoff across files, but within a file, you can copy and paste to your hearts content.
Matthew Anderson, PE
Matthew Anderson, PE CFM
Product Manager
Autodesk (Innovyze)