Can I mix two cpus with a different clock speed on a board?
This could go either way, as the socket and stepping may be the same, but some systems are sensitive to the difference. E.g. I've inadvertently had "CPU mismatch detected!" errors on server systems with CPU's of slightly different specifications.
In your best-case scenario, your system will clock down to the speed of the slowest CPU. Worst case, the system will halt immediately.