The X2.6 chip has a switch between banks 1 and 2. bank 1 is comprised of holding 512k bios. If you use the X2.5034 or whatever bios, that is 512k already.... if you use anything else then its prolly gonna be 256k so you have to put 2 of them together.
Switch on bank one of your chip = 512k bios
Switch on bank two of your chip = 512k bios
so when you make a 1 meg bios, it should flash it to all of the banks if that makes sence. Best thing if your still confused is to post which bios you have and i can show you exactly how it should be put together