When I tried two custom humans yesterday, the second instance could not get past the loading screen, and the host instance loaded in with control of the character he created, but only the portrait of the second character (who never loaded in) was shown. I got it working by mixing a custom origin with a story origin (Custom: Human and Lohse in this case).
I just tried two custom humans again today and both were able to load in. I also tested 4 local instances, and that worked when all 4 characters were different custom races.
