Two different Intel xeon e5 24xx on one dual-socket motherboard

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.

See: Is it possible to influence the way CPUs are enumerated under Linux?

and: Can I mix two cpus with a different clock speed on a board?

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."

http://www.intel.com/content/www/us/en/processors/xeon/xeon-e5-1600-2600-vol-1-datasheet.html (page 165)

http://www.intel.com/content/www/us/en/processors/xeon/xeon-e5-v3-datasheet-vol-1.html (page 28)

http://www.intel.com/content/www/us/en/processors/xeon/xeon-5500-vol-1-datasheet.html (page 25)