For better undestanding of the situation your system's having, please, provide more details:
1. What's the serverboard you're running the two Intel(R) Xeon(TM) Processor(s) X5680 on?
2. Just to be in sync., what do you mean by siblings? Are you referring to the threads?
3. I checked and there's no Intel(R) Xeon(TM) Processor X5500; see the following URL:
Hence, please, confirm on the model of this other Intel(R) Xeon(TM) Processor.
4. Have you run the Intel(R) Processor Identification Utility from DOS to see how the cores are being detected?