@Alistair: if the gpu-manager detected only one GPU, it means that the
other one was probably powered down (or it hadn't been powered on yet?).
Either way, you can work around the problem by passing the
"nogpumanager" kernel parameter on boot. This will disable the
gpu-manager. If that helps, you can make the parameter permanent in
GRUB.
@Alistair: if the gpu-manager detected only one GPU, it means that the
other one was probably powered down (or it hadn't been powered on yet?).
Either way, you can work around the problem by passing the
"nogpumanager" kernel parameter on boot. This will disable the
gpu-manager. If that helps, you can make the parameter permanent in
GRUB.