How to boot from a backup bios chip?
Unless the motherboard supports an auxiliary BIOS chip, you will need to perform surgery on the motherboard PCB. SPI requires a separate chip select for each slave device.

Without explicit support, you need, at a minimum, to disconnect the CS pin from the on-board device. This might mean lifting the pin of the IC or cutting the trace (which might be buried in an inside layer of the PCB).

If you need this, I’d highly suggest buying a mobo with explicit support.