This issue is being tracked as SPR TMS-248
Delta of 2 Documents should be available from the Actions menu in the Notes client when 2 documents are selected in a view.
If the option is greyed out then check that 2 documents are selected.
There is a known issue being investigated where the correct context is not getting returned within the frameset navigation. As a result, you will have to get out of the frameset and navigate to the view with the outline and use the feature there.