Xeon X5660 with Xeon X5560

[Exusiai]

I am building a dual processor server which run two xeon cpu, I already have one X5660 running in my server. Can I put a x5560 in my server without any compatibility problem?

 

[velocityg4]

The simple answer is a dual processor motherboard needs two of the exact same model of CPU.

The long answer is there are a few exceptions. But your CPUs definitely do not meet those exceptions
From MU_Engineer: http://www.tomshardware.com/forum/272899-28-different-processors-dual-socket-motherboard
1. CPUs must have the same QPI and RAM speed to work together.
2. CPUs must have the same thermal profile (TDP) to work together.
3. The CPUs must have the same number of physical cores to work together.
4. The CPUs must have the same number of logical cores to work together.
5. Stepping does not matter.
6. Clock speed does not matter.