This discussion has a sample that will get you started:
The Restricted Navigation does not show the link to the next section until the previous section has been viewed. The Mixed Navigation allows the various sections to be visited in any order, but does not allow going to the End until all sections have been viewed. The Free Navigation allows the sections to be visited in any order, and the End to be visited at any time.
I would think you can use the Mixed Navigation. Let them choose either option, but once started do not allow a return to the menu slide until the chosen option is finished. Then after both are completed, instead of jumping to the end, jump to the next choice slide.
