Bit of a nitpick, but I think you meant NetBSD/OpenBSD? AFAIK both FreeBSD and NetBSD independently continued the abandoned 386BSD project, but there was never any strong relation between the two projects (they did share code later on). OpenBSD did split from NetBSD, after Theo de Raadt was removed.
Other examples: FreeBSD/DragonFlyBSD, Vim/NeoVim, Mplayer/mpv, (Open)Solaris/illumos, MySQL/MariaDB, probably some more.
https://stackoverflow.com/questions/27309412/what-is-the-dif...
There are loads of forks that happen due to disagreements in organisation.
> Dillon started DragonFly in the belief that the techniques adopted for threading and symmetric multiprocessing in FreeBSD 5 would lead to poor performance and maintenance problems. He sought to correct these anticipated problems within the FreeBSD project. Due to conflicts with other FreeBSD developers over the implementation of his ideas, his ability to directly change the codebase was eventually revoked.
Do SQL dialects in Databases count? mariadb