This approach is really just a combination of the first two methods, it just takes a little bit of editing/cutting using the tools that SnagIt offers you after you complete a recording. You wuld just cut out the parts where you transition from the presentation to the demonstration.
For this method, you might want to first walk students through a presentation, this could be where you explain important terms and/or setting up the context of the video. Personally, I use this to "set the stage" for the demonstration that would follow.
This method is fine if your video is concise....but you should consider whether or not to breakup your presentation/demonstration into a series of individual tutorials. Video #1 could be your presentation, Video #2 could be your demonstration. Breaking them up like this has been found to be a best practice and more effective.