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?
My mistake, I jumped the gun on this. The test was inadvertently done on a 32bit Linux kernel. When we switched to the 64 bit Kernel /proc/cpuinfo shows 24 "processors" as I was expecting.