I have Dell precision 690 with dual xeon x5355 and I want to use this workstation with single cpu configuration. I googled but no success. Years ago someone told me that dual cpu mobos can run a single processor with a dummy processor installed in second cpu socket.
No such thing as Dummy processor. There are socket covers that are recommended when only 1 cpu is used. Also if there is a Riser card for the 2nd cpu this would need to be removed. The CPU 0 needs to stay.