All ASRock, GIGABYTE, MSI H55 mb I tested support the simultaneous use of onboard video and a discrete graphics card in the PCI Express x16 slot.
The manual of Shuttle SH55J2 says, "Note, that the integrated graphics will be disabled, while using a discrete graphics card." So SH55J2 may be similar to ASUS and Intel. Check also AVADirect
(you can select a PCIe x1 TV tuner card).