Can I use two different e5 24xx on one dual socket motherboard or must the CPU's be identical?
Just don't do this.
It doesn't matter what the silly reason or specific circumstance is... Either use one processor or two identical processors.
Yes, you can. Intel says in their Xeon Datasheets that you only need to match the:
- Intel QuickPath Interconnect frequency
- core frequency
- power segment
- internal cache sizes.
Also, they state "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."