[jarvus lunt]

There are some various ways to handle this inside of RAS. What you need to do is not very obvious and it depends on how things are set up.

When you say two different reaches, do you mean two different reaches in the same geometry file, or in different geometry files, or in different projects?

Is the river stationing of the two different reaches consistent (and do they have the same reach name and river name)?

One approach you might look into, on the geometry editor under file there is an option to Import Geometry Data... You can use this to bring in geometry data from a different geometry file (the geometry file can be from the same project or from a different project). If you you import cross sections with the same reach and river name, RAS will assume that they are part of a single, continuous river.