To merge CHM files at run time, you must designate one of the projects to be the main project (master); the others are subprojects (slaves). In the configuration file for the main project, in the [HelpMerge] section list the names of all the subproject CHM files to be merged, omitting file extensions. For example:
[HelpMerge] LibRef AdvModule HelpOnHelp
The merge process includes any subproject’s [HelpMerge] data; as a result, any other subprojects specified for merging into a given subproject are also integrated into the main project, allowing any degree of nesting of subprojects.
Place a HelpMerge PI marker in your main-project DITA document for each subproject listed in the [HelpMerge] section, to show where the subproject should be merged into the main project, and to specify a contents level for the top TOC entry for the subproject.
Insert the HelpMerge marker between two main-project topics, in either of the following places:
The content of the HelpMerge PI marker consists of a single-digit contents level for the top TOC entry, followed by a space, followed by the CHM file name of the subproject, without extension. For example:
2 HelpOnHelp
For more information about merging multiple CHM files, see Creating Help > Manage Large Document Sets in HTML Help Workshop Help on HTML Help.
§16.4.3 Setting contents levels for HTML-based Help
§16.5.7 Specifying index link destinations for HTML-based Help
§18.14.5 Comparing HHW settings for stand-alone vs. merged CHMs
Rob Chandler’s Web site: http://helpware.net/htmlhelp/how_to_merge.htm