Generally
Split board, sometimes refereed to a 16th edition board, is a CU with a RCD protected side and a non-RCD (main switch) side. Sockets (and occasionally other circuits e.g. showers) would be on the RCD side of the CU and non-RCD (e.g. lights) protected circuits would go on the main switch side
Dual RCD, sometimes called a 17th edition board, is a board loaded with two RCDs (+Main switch) and the majority of circuits distributed between them (as 17th edition regs require just about everything on RCD due to burying less than 50mm, in domestic anyway). For discrimination purposes Upstairs lighting+downstairs ring will be on one side and downstairs lighting+ upstairs ring, this is also so if one RCD trips you should still be able to see on that floor.
Personally I still like to use a split board with additional RCBOs to allow for better discrimination but keep the costs lower than a full RCBO board.
Hope that helps but by the time I've typed this somebody will of probably come up with a better explanation :builder: