Many of the cores locked on such CPUs were locked, because they were not working as they have to.
Sometimes they don't work at all, sometimes they don't run stable. Voltage leaking can also be an issue due to which these cores may have been to.
Increase the voltage by slight steps and keep an eye on CPU temperatures.
Run Prime95 or equivalent software for a few hours to clarify that every core is running stable on full load. If you can't get all of the cores to run stable at acceptable temperatures, but only one core is failing, you can try to disable this particular core in your BIOS/(U)EFi.
Also, running a CPU with more cores almost always results in a higher required voltage, so this may be normal.
As long as your voltages stay below 1.4 to 1.45 volts and the CPU's not getting too hot, you should be fine.
I would consider you to either obtain a native quad-core CPU instead of unlocking your dual core. Core unlocking is a nice feature, but unfortunately it's not working on all CPUs that have cores disabled.