Intel recommend using two identical CPUs in their socket-2011 dual-socket motherboards; it is theoretically possible to use non-identical CPUs, though, and Intel’s rules for that are that they must be:
- Of the same processor family
- Have the same number of cores
- Have the same cache size at each level of cache
- Able to find a common QPI link frequency
Given the above, Intel states that you can mix CPUs with different core frequencies – the faster CPU will clock down to match the slower one, however. The same applies to the QPI links.
We haven’t yet had the opportunity to test differing CPUs in other brands’ dual-socket motherboards yet – we will update if or when we do. This isn’t something which we would usually recommend without finding someone who has successfully tested your intended combination just in case, but it is handy to know that it is theoretically supported.