Android - Render 2 videos into 1 with Watermark

My current app is developed in Android Studio and records 2 mp4 videos, then plays them back in sync with audio. I need a coder with experience in Android NDK, ffmpeg or similar libraries to add functionality to the existing app. The function is to 'mixdown' these 2 (320x240) mp4 videos to a single (640x360) mp4 video (including both audio tracks) with a watermark on top. The final video will have the dimensions of the attached jpg. UI is not important, only the audiovisual 'mixdown' functionality. Please make sure you have good english communication, relevant experience and a clear understanding of the requirements before promising to complete. Reply with the code VP2in12 in the first line of your reply. thanks.