Gigabyte boards are dual BIOS. They are great and the main reason that I only purchase Gigabyte boards for builds. If your BIOS gets corrupted the backup BIOS takes over so you can access the bios and flash the new BIOS which is real easy with Gigabyte. You don't have to send the board out to the manufacturer. You can save your settings so you do not have to go thru all the settings again. I have a regular setting and a gaming setting. When I intend to play a game, at bootup I enter BIOS and choose the gaming setting and vice versa.
You can write to both. If I upgraded my BIOS to UEFI, I have read that I would need to upgrade both (no plans to do so though). It is a great idea, I think, in case something goes wrong with an upgrade.