ArcadeStriker
map layout design is my passion
If this does happen every time and it seems that its because the computer has trouble running both instances at the same time (or if you notice that with one its already laggy, it likely will be even more with two)...then that might unfortunately be all that it is. That said, I sometimes also got some occasional lagspikes that made the other instance leave, but it works when I try joining with that same instance again, though in that case it doesn't crash the game but more like it disconnects from the main instance server.
If you are using the CMD window and are in the SRB2Kart folder, just using that command (the game exe + the port command and a different port; you can use "srb2kart-moe.exe" if you're using the Moe Mansion exe and such) should do the trick; for a batch file its the same but creating a batch file is probably harder than it should be if you have visible known file extensions disabled in your PC (this is if you don't see ".exe" and such extensions on the filenames), but if you do have it enabled, you can just create a text file, rename the extension to .bat, open it with Notepad, and put the command in there. After that and saving it, you open the bat and it should launch the game with the different port set.
