ISCC between two SIP servers

Are your SIP Servera connected to one another? Each SIP Server Application object needs to reference the other in the connections tab.

How did you go with this?

See this:
Line 487: @18:28:47.7270 [ISCC] Received from server SIP_Server@SIP_Switch/1: message ISCCEventAuthorizeResponse
Line 487: @18:28:47.7270 [ISCC] Received from server SIP_Server@SIP_Switch/1: message ISCCEventAuthorizeResponse
Line 488: ISCCAttributeResponseCode Not_Configured
Line 489: ISCCAttributeReferenceID 6619138 [00650002]
Line 490: ISCCAttributeTrackingID 6553605 [00640005]
Line 491: @18:28:47.7270 [ISCC] Error: Server SIP_Server@SIP_Switch/1 has rejected the connection. Reconnecting…
Line 492: @18:28:47.7270 [ISCC] [connection 0000000002a41810] disconnected [628]2

I am confused - is one your backup, or are you doing multi^site sharing between the two.