This is a small example package showing how to build a simple player that displays two independant playlists side by side. Should be easy to change if you need other layouts.
Playback works by preloading the next asset while the current one is playing. This means that at most 4 videos are loaded at the same time.
You should not attempt to play two FullHD videos next to each other. Doing so might be too much for the Pi and it might cause a lost video signal as the Pi cannot generate and HDMI output signal fast enough. It's best to use videos/images that exactly fit into the available space.