Hi svntwoo, Since the two cores only seem to shutdown during idle, and otherwise function fine and reach similar in use temperatures as the other cores, that is a clue.
I don't own a Sandy Bridge CPU PC so I can only speculate on the details of this. In your BIOS or UEFI, find and see what CPU C-State is set to, that would be likely be in the CPU or Power section. C-State settings are power saving options that can function at a CPU level or a CPU Core level. The higher the number after the 'C', the more power savings, and the lower the CPU or Core is slowed down until its core clock is literally shut off when in the lowest C-State (C6 or possibly C7, etc.) There is probably a general C-State enable/disable, and a C-State level selection (C1, C2, C3, etc.) that allows you to choose the lowest C-State the CPU or Core will be allowed to enter.
It appears that those two cores are entering a low (or is that high?) C-State level and are or almost are completely shutting down. You can test my C-State theory by going into your BIOS/UEFI and disabling CPU C-State. Then monitor the activity of the cores at idle as you did previously. If they aren't at that low temperature anymore, then you have your answer. If they still do so, then we'll need to check something else.
Your CPU is not being damaged or running incorrectly when the cores enter a "deep sleep" C-State when the PC is idle. That is normal when CPU C-State is enabled. You may be able to set the level to stop at a C-level in the middle of their range so the Cores don't sleep as deeply, although all that does is show you a higher Core temperature since the Cores are more "awake", plus use more power. If seeing those non-equal temperatures bothers you that much, you could adjust the C-State level to eliminate that.
Please note that when you disable CPU C-State, you may also lose the EIST power saving function, which I believe is actually the C1E C-State, or associated with it. That may be different with Sandy Bridge CPUs so I am not certain.