SMARTID+ only the connected id can be identified.

I have two SMARTID+ units and can register both and id each one when it is directly connected to the ONX-580, but when I try to conduct a test with the second SMARTID+ connected to the first with a coax connection, it says there is a bad connection at the far end and it can only id the SMARTID+ that is connected to the ONX.

You cannot conduct testing between two SMARTID+ units. The far end id needs to be a SMARTID and not a SMARTID+. The SMARTID+ is the master unit that gets connected at the ONX. All remote ids connected to the SMARTID+ should be regular SMARTID units.