An easy way he might have done it, was to put the slow mo where he wanted it, send it through ICloud photo sharing to you're computer, copy it out of the ICloudPhotos sharing folder (Pictures\iCloud Photos\Shared\Family). Then go back to you're iphone 6, put the slow mo at a different spot, and then again send it through ICloud photo sharing to you're computer. Copy it out. Now you will 2 of the same videos, but with a slo mo at different times.
Use edit software to put them together at a interacting point.
Here is one I did using my iphone 6, took me about 8 mins.
https://www.youtube.com/watch?v=Djbr7qdT5fs&feature=youtu.be