Hi there,
At this point, we are not planning on adding multiple-player support to Lumen beyond the split-screen mode. Some other higher-priority features are currently being worked on, but I have filed a feature request with the engine team to let them know you are interested in this addition. As for your second question, I have also asked the team about how feasible it would be to implement this feature yourself and will get back to you once I have some updates.