This line in dthelpview's application-defaults file (DtHelpview) specifies the help on help volume:
DtHelpview*helpOnHelpVolume: Help4Help
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);