Windows 10 does not see second CPU

I have Dell Precision T5400 workstation with two Intel Xeon processors. One a x5450 (quad core) and a e5450 (quad core). Both CPUs are recognised in the BIOS

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I can not even install Windows with 2 processors installed. First I have to pull out one of them, then install windows. If I put the second processor back in I get error 'Multi processor not supported'.

I can work around that by booting with a single CPU and in System configuration -> Boot advanced options select the checkbox 'Number of processors' and set it to four (4 because that is about processor cores).

After that I can start windows and it wil use a single CPU.

Note that Device manager see 8 cores (2 CPUs).
Task manager see just 4 cores (in performance tab).

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I installed Windows 10 Pro (updated from 8.1 Pro), and OS see just one processor.

Does anyone knows what is the problem?

Answer

The problem was been that I had 2 different CPUs, one is X5450 and another is E5450. When I changed one of them, to be the same, problem is fixed.