Peter once you put your video together, in order to play it in any other form than in Power Director you need to produce it. You will then have the option to choose mpeg1 or mpeg2 avi etc.
When you say you want to add another video, do you mean edit so that the two videos are one long video? If so, then you add the second one to your timeline in Power Director. Even then, in order to watch them in any other format than Power Director, you need to produce the video (that takes it from a .pds (which is essentially a working file) to a video file of some sort.
Hopefully I have understood your question. If not, please clarify further.