Yes, you can upload more than one game. We didn't quite think that participants will be able to make 2 games in a 1 week time period. But there are very talented people out there. So why not? It will improve your chances too!
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Your concern is valid. But we are looking for games that can run on mobile devices. This way we can test how well the touch controls work, how well the game is optimized to run on lower specifications among other things. If you are worried about plagiarism, well someone can just copy your game idea and make one for mobile devices. It's up to you :)