The darker area maybe carbon spots developing. Sometimes they appear early, sometimes many years later. Nothing can be done with them to fix the issue. Keep the better one. These maybe from the same die? Possible master die doubling on them:

You could check the other areas shown in my images. Not premium for master die doubling as 1/2 of the coins struck from all three mints had this issue. So when 50% of the coins are affected, then they are just considered normal coins. In your case, a normal proof coin.