Almost all CORIO products will output a YUV signal along with separate syncs (H & V) on the HD15 connector. This is because the signal is from the same chip as the RGBHV signal.
However, if this YUV+HV signal is linked to another CORIO unit's HD15 input, it will usually have problems detecting what signal is present - since two syncronization signals will be present (sync-on-Y and H/V sync).
To avoid this issue, go into the 'Adjust sources' menu and manually force the 'Input type' to 'AYUV' (analog YUV). Alternatively, if the CORIO unit supports it, use 3xBNC connections instead.