The main difference between the B760M Gaming DDR4 Single BIOS and the B660M Gaming DDR4 Dual BIOS is the number of BIOS chips and their capacity.
A "BIOS chip" refers to the chip on the motherboard that contains the BIOS (Basic Input/Output System) firmware, which controls the basic functions of the computer, such as booting up and interacting with the hardware.
The B760M Gaming DDR4 has a single BIOS chip with a capacity of 128 Mbit (16 MB), while the B660M Gaming DDR4 has two BIOS chips with a combined capacity of 256 Mbit (32 MB).
The Dual BIOS feature provide a backup if one BIOS chip fails, the system can still boot up using the second BIOS chip, which can be useful for overclockers or system builders who are experimenting with different settings and run the risk of corrupting the BIOS.