For those who want to do 2 player speed runs, I have created a discord server. https://discord.gg/PSF7q6ka
So, I've been grinding on two player 150cc speed runs to prove viability for around three months. Can't speak to three and four player runs, but two player runs are a completely different animal than one player. It's co-op play, and I'm fairly certain that it could be faster than a one player run. First, take into account items. You now have four items to work with. Which means you can block most of the third place hits. You can snipe blue shells from second place. There's lots of coordination between players that has to happen and many many failure points not present in 1player. Knowing coin lines, but not taking them once you have 10 coins, so second can get the coins. You have to have two players running identical set ups, with similar skill levels. When I first started this, I was getting 2h18m runs. Got it down to 2h6m flat on cartridge, or 2h5m with 1min removed for cartridge play. I have plenty of room to continue to improve these times. That's top 200 vs 1player speed runs. You have to worry about accidentally hitting your second player. It's really complicated. I'd suggest trying it with two players and seeing how you fair. It's definitely a different category, and honestly has been brought up dozens of times, but I don't see anyone really trying it full time, outside of my play.
