UP Core + High speed + Low Speed carrier board

Hi,
I'm in weird situation. I want to connect both carrier boards to UP Core but it does not work. Simply - my UP Core does not recognize second connected carrier board.

If I connect both carrier boards then only one is recognized in BIOS. It does not matter in which order I connect them. When I connect only one of carrier boards, then both of them works correctly so the carrier boards may not be faulty.

Does anyone have any idea what might be the problem?