Help with Portalite Elinchrom 500

I just bought a pair of identical lights, Portlite Elinchrom BX 500RI.

I set them both up right out of the box and didn't change any factory fsettings, but I noticed that one light won't work unless the other one is on.

They are both identical but why would one of the light can't be use by itself?   When both lights are on they both work fine.  The one that doesn't work when the other one is not on, if I press the test button, you can hear it firing like it wants to flash but no flash unless the other one is on.

Can someone help me?  Looked every where in the tiny manual and couldn't find the answer.

Thanks for any help