mind PM'ing me the two forums your not naming please? i'm a curious induvidual.
The "major" forums as I know of can be summed up as Space Battles, Questionable Questing, Sufficient Velocity, Frozen In Carbonite, and The Sietch. Space Battles is the originating site for all the others is itself a spinoff from StarDestroyer.net back in the mid 90's as Johan Alm's CG movie site and the forum taking a life of it's own.
Questionable Questing is the first big SB sister site made from all the people who didn't understand why you couldn't roll play loli porn in Creative Writing on SB. The old owner had banned NSFW material which is held to this day by SB's staff. They made QQ to have their own forum with blackjack and hookers, hold the gambling and extra hookers please. Now we have the lewds. People who can't post stuff in SB/SV for being to explicit or looking for porn written by writers on the other two come here.
Sufficient Velocity popped up when a big name mod got fired from the staff but the admins tried to make it look like a mutual parting then got called out. The outraged userbase with several year of slowly failing software and database problems with the owner being MIA and unreachable for all intents and purposes meant when Squishy launched his clone and made porting over archives easy at the height of the scandal a lot of writers and questers cloned over all their works in the first few weeks and months giving Sufficient Velocity a huge chunk of SB's userbase and works from the start. As the SV rants thread will show you still have the problem of Squishy and his lawfare leftist based site management but the site is stable and self substantiating outside of the hot mess that is all of General Discussion. Without the scandal and site problems to jump-start SV it would be a far smaller site today with nothing to really draw SB users.
Frozen In Carbonite is such a site started by a former member of all three Horton. He got banned from the triad so made his own. There's really nothing to draw membership for his site other than guy's banned from the big three can join but most of them got banned for a reason. All of SBs issues that made it look like the site was failing were fixed by then and the other two had no scandal to capitalize on nor was he such a big name to draw a crowd with his rep. It's all but dead in traffic but enough people know about it and are from the big three to count as a spinoff.
The Sietch is the latest splinter site to come from the SB community. The latest big scandal was a long running mass PM group composed of right leaning member who kept getting in trouble with whitehall debates and some of the mod staff. Somebody found out about it and dragged up the worst posts they could find while cropping dates and names to make it look like a Nazi infestation in SB was going on. A huge mess splattered out of it and some people from the PM got banned. They founded The Sietch as a site more accepting of right wing views than the greatly left leaning SV and non political QQ would allow. Of course this means most of their traffic and posters are there for right wing stuff that runs into Rule 8 for QQ so anything to deal with them is like linking QQ on SB.
Other than than I'm sure there's a dozen more splinter sites from SB over the last two decades but either never got off the ground or are so long ago I've never heard of them. In general SB is the main hub, SV the left leaning clone, QQ the lewds, Sietch the rightys, and FiC the deadbeat Chihuahua.