I want to create a splitscreen video from two video clips. Left one is of duration 70seconds. Right one is of duration 80seconds. In the splitscreen video, the right video should start first. Left video shouldn't start for first 15 seconds. After 80s, i.e. after right video ends, left video should continue. So, the final splitscreen video should be of 85 seconds. For the first 80 seconds, the audio should be from right video and for the next 5 seconds, it should be from left video. Or vice-versa is also fine (i.e. first 15 seconds, audio from right video, next 70 seconds, audio from left video). How to do this?
Another optional requirement: For first 15 seconds, when right video is playing, in the left part, instead of black screen, it would be preferable to have the first frame (image) of the left video. So, after 15s, when left video starts, it takes off from there. Similarly, after 80s when right video stops, instead of black screen, it would be preferable to have the last frame of right video there.
Edit: Opensource software is preferred.