Dual Bios means double bios installed on a gpu. You might have seen there's a button on your gpu, one is to use the second bios which is especially for overclocking the other is for normal gaming at base clocks. There's also a button to flash the bios, just in case we mess up with them, it brings them to default settings. Yes, it enables boost clock. And again yes, dual bios can be operated with legacy and uefi bios.