Is the device manager able to find it? Check it... Device manager is in the control panel. I'd recommend you to un-install any drivers currently installed and then re-install the updated drivers.
No, it only sees one GPU, same in Linux.
Try to take out the one which is working, and place the one which is not working in that slot. See if it is showing any display at all. That's a sign of a ... dead GPU by the way. Hopefully it's not dead.
Can the GPU be taken out? I thought they were soldered in.
I will try that, do you which one is slave? The one above the PCI-e connector?
GPU's can be taken out very easily, with a bit of force. Take out both of the GPU's, and try checking and making them work one by one. Just don't place both of the GPU's at once. Google around any tutorial/video if you're taking it out for the first time, it's easy.