The answer is that particular combination won't work together. Page 25 and 26 of the Intel Xeon 5000 Series Datasheet Volume 1 discuss mixing processor types. A brief rundown on what's supported and not:
1. CPUs must have the same QPI and RAM speed to work together.
2. CPUs must have the same thermal profile (TDP) to work together.
3. The CPUs must have the same number of physical cores to work together.
4. The CPUs must have the same number of logical cores to work together.
5. Stepping does not matter.
6. Clock speed does not matter.
If all of those are met, then you can run the CPUs together. The CPUs run at the speed of the slowest CPU. The current mix and match list:
1. Xeon E5502- can only work with another E5502 since it's the only dual-core Xeon 5500 (#4.)
2. Xeon E5504 can work with the E5506, both CPUs run at 2.00 GHz.
3. Xeon E5520 can work with the E5530 or E5540, both CPUs run at 2.26 GHz in either case.
4. Xeon E5530 can work with the E5520 (CPUs run at 2.26 GHz) or E5540 (CPUs run at 2.40 GHz)
5. Xeon X5550 can work with the X5560 or X5570, both CPUs run at 2.67 GHz in either case.
6. Xeon X5560 can work with the X5550 (CPUs run at 2.67 GHz) or X5570 (CPUs run at 2.80 GHz)
7. Xeon W5580 can work with the W5590, both CPUs run at 3.20 GHz)
8. Xeon L5506- can only work with other L5506s since it's the only L-series Xeon 5500 without HyperThreading.
9. Xeon L5518 can work with the L5520 or L5530, both CPUs run at 2.13 GHz in either case.
10. Xeon L5520 can work with the L5518 (CPUs run at 2.13 GHz) or L5530 (CPUs run at 2.26 GHz.)
11. Xeon L5508- can only work with other L5508s since it's the only 38-watt TDP Xeon 5500.