Dell Poweredge R410 does not recognize second CPU

I have a Dell Poweredge R410 with two CPUs installed.

I tried everything but the second CPU does not get recognized by the BIOS or the Linux Operating System it has.

Both CPUs are Intel Xeon E5602 with these RAM configurations:

- CPU1 - 16GB ram (2 8GB DIMMs in slots 1 and 2)
- CPU2 - 4GB ram (2 2GB DIMMs in slots 1 and 2)

Couple of extra info:

1. RAM is working fine (I can get 20 GB of RAM if I use all CPU1 slots).
2. Both processors are also working fine. I can make work either one of them as the main CPU.
3. Both heat sinks are working fine.
4. No hardware failures when running Dell's hardware check.
5. If I use one 8GB DIMM on each CPU, it doesn't work either.

Is there anything else I need to setup? Manual doesn't say anything about a JUMPER and/or BIOS configuration needed in order to make both CPUs work.

One more thing: firmware, BIOS and everything are fully updated to their latest official versions.