Physical keyboards often have a limitation on the amount of certain keys that can or cannot be pressed at the same time.
As for fix... you can test and try for a combination of keys that do work, but there is no guarantee that it will work on another keyboard. As the Wikipedia article mentioned in the game Starcon, there was a system where the game tested for conflicting keys as you configured the controls, but that would be a pain to implement I think...
You can plug in an extra usb keyboard, that should solve the issue. Or otherwise use separate input types for each player such as mouse or gamepads.