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As you are my friend, it is a pleasure that you liked it. Well I used an editing program called Sony Vegas, version 11. But there are applications for cell phones where you can make the effect and easier as viva video or movie maker.

Okay I think I have viva video but don't understand how to use it well

Easy my friend, just choose the two videos, trim them so that they are of equal size, control the timeline so that both go at the same time and that's it my friend. There are many tutorials on youtube where they explain it better hehehehej, with words I find it hard to tell you. I hope I helped you

Okay I will check the tutorial out on YouTube but you still did a lot by telling me