The only universal rule I know is that all CPUs in a DP (Dual Processor) configuration should be exactly the same revision and stepping. For example, in the Intel D5400XS motherboard manual, it specifically states in section 1.4 that "Processors installed must be identical
as determined by identical sSpec numbers.": http://download.intel.com/support/mothe ... 8001us.pdf
If your server docs don't specifically state what is or isn't supported, then you'll need to consult the processor docs, which are often fairly specific about mixing and matching DP capable CPUs. Intel has a general reference on the subject here:
If you read this carefully, you'll see that they basically state that mixing steppings is generally OK, however mixing speeds or other attributes is not. There are exceptions though, such that unless you have a reference that states a mix is OK, the only sure thing is if even the steppings are the same. You didn't say which type of processor your server supports, but for example the Xeon 5400 series datasheet http://download.intel.com/design/xeon/d ... 318589.pdf
states (in section 2.11 - Mixing Processors):
"Intel supports and validates dual processor configurations only in which both processors operate with the same FSB frequency, core frequency, power segments, and have the same internal cache sizes. Mixing components operating at different internal clock frequencies is not supported and will not be validated by Intel. Combining processors from different power segments is also not supported..."
"...Mixing processors of different steppings but the same model (as per CPUID instruction) is supported..."
So I guess for *that* CPU, you can get away with mixing different steppings.
Bottom line: Unless your server or processor documentation specifically states otherwise, I recommend using chips with the same revision and stepping in DP (Dual Processor) configurations, however the mixing of different steppings is supported in many cases. In cases where steppings are mixed, I don't know of any rule about which should be placed where, but I'd probably go with the lower (earlier) stepping in the first socket as a common sense rule of thumb. Let me know if you find any official word more specific to your server and processors that might give a more precise answer. Scott.