Well in my experience of dual bios motherboard, the switch just toggles which flash IC is enabled via its chip select pin. Both ICs are on the same SPI bus just that one is disabled while the other is active. So as long as they decide to store the efivars on the same IC as the uefi its going to be fine.

I mean the whole point of dual bios is for disaster recovery so not having separate storage space while having dual bios seems a bit pointless imo. But tbf, doing something like that wouldn't be the most stupid thing I heard a manufacturer do.