if you want to split your 256mb dump into 2 separate dumps then download these donor files
http://www.ps3devwiki.com/files/flash/dumps/NAND0-1.rar and then use flow re-builder and use flash0 as your top nand and flash1 as your bottom nand then use your 256mb dump as your unified nand then click execute and it will create 2 flash-able dumps