Off camera dynalight question

Here is my problem


I'm using a canon 50D and I have no problem syncing my 580exIIs:one on and one off camera and using infra red. It works. Not great but it works,


Now I want tochange the off camera 580 with an off camera dynalight unni400jr (strobe light).


I start by taking the 50D and adding a pc connect to the dynalight. It works like a champ.


Ok here goes now I want to add back a 580exII ON CAMERA and use it in manual mode.


The 50D manual page 114 at the bottom states "the hot shoe and the pc sync can both be used together. "


So why does the dynalight not fire with the on cameraflash isattachedandON. The 580 fires but the dynalight does not.
Do i need to set something in the 580 or the 50D menu??



For sake of simplicty I'm leaving the dynalight attached via pc cable