Tell HTML Help Workshop what CHM file you are using as the default:
[MSHtmlHelpOptions] ; DefaultChmFile = name of .chm for project if not in [ChmFiles] DefaultChmFile = MyProj
See §18.3.6 Naming project and compiled files for HTML Help.
DITA2Go uses the default CHM file name as the destination for any DITA files that have not been mapped to other CHM file names; see §18.14.1.2 Mapping DITA files to CHM files.
To support cross references between the current CHM file and CHM files from projects in other output directories, see §28.6 Linking to other files and other DITA2Go projects. To support interproject hypertext links, see §18.6.3 Linking to external files from compiled HTML Help.