Add a line to your application's application-defaults file like this:
Where App-class is the application's class name and volume is the name of the help on help volume you want to access.
Or, within your application, set the helpOnHelpVolume resource for each help dialog you create.
This line in dthelpview's application-defaults file (DtHelpview) specifies the help on help volume:
To specify the help on help volume when creating a help dialog, add it to the argument list passed to the create function as shown here:
ac = 0; XtSetArg (al[ac], XmNtitle, "My Application - Help"); ac++; XtSetArg (al[ac], DtNhelpOnHelpVolume, "Help4Help"); ac++; helpDialog = DtCreateHelpDialog (parent, "helpDialog", al, ac);