As one of the people still interested in both, I can say I think the split is a decent idea.
The pro's for it are:
1. Less stickies- when I was moderating this forum I hated how many stickies we had. Your project could be getting replies every ten minutes but you are still halfway down the page. I'd move the modding guild over with the split, I'd unsticky the CSC archives (they are linked in the main CSC thread), and unsticky contests (they stay near the top anyway). For that matter I'd unsticky the CSC. Threads that remain very active look better when they are being topped up than stickied. But this is just my opinion. At the least, the split would allow for two or three modding dedicated stickies without further cluttering the top of the SD forum.
2. Read what you want - No more, 'this looks good but I know nothing about modding' comments. Or conversely, Suggestions to mod vanilla campaigns. People hunting for SD threads don't have to sift through total conversions.
3. SD projects stay on the first page longer. Once topics hit the second page, they sort of die.
4. Dedicated modding forum staffer - Someone who knows the community and can organize modding contests or community projects. Dire Wolf already brought this up but it's worth repeating
The con is you divide activity between two places so neither looks as active. However, it would result in slightly increased activity overall I think.
We merged the two forums back when I was staff because the modding forum had basically died. There was a reason for that though. Modding the game data was so incredibly time consuming with Genied1 no one could get into it. Graphically, most modding consisted of ripping graphics from other titles and by 2006 or so everything had been picked over. Now, people can make total conversions and change almost anything. There is orders of magnitudes more graphics on the blacksmith to pick from if you can't make your own. SD might have declined, but the modding community is better off than in AoKH's so called glory days.
With SD, there is no activity, or not enough. Merging or splitting isn't going to help that. Measures need to be taken to increase activity and splitting isn't going to help. It might help the staff focus their attention on it, but that's about it.
TLDR, I think the split is a good idea for visual and practical reasons, and will benefit modding. It's going to do little for SD, so if this is being seen as a solution to the decline in design interest, I don't agree with that.
"And Matt is a prolific lurker, watching over the forum from afar in silence, like Batman. He's the president TC needs, and possibly also the one it deserves." - trebuchet king