It has taken us Marble Blast Platinum devs 3 whole years in order to make multiplayer as successful as it is. A ton of time and research was put into this. Thousands upon thousands of hours. You need expert programmers who are experienced in TorqueScript and handling networking code. HiGuy and I invested months into this. You have marble movement, interpolation, collision, syncronizing the clients across a network, moving platforms, bugfixes, port code in stock MBG from single player to multiplayer and other things. That isn't even getting to a game mode yet. Let's not forget about scoring, leaderboards, chatting with one another, ect.
This stuff takes thousands upon thousands (yes thousands) of lines of code to accomplish this kind of stuff. How much do code do you think higuy and I had to add to MBP 1.50? About at least 100,000+ lines of torquescript, and that is a very very conservative estimate.
If you want to code multiplayer, go ahead, but there's so much more than just collecting gems and getting the most points.
I am a programmer. Most here know me for being one of the major contributors to Marble Blast Platinum and PlatinumQuest.