In other words, contents of left hand is 50/50 for Head or Tail, Content of right hand is also 50/50 for Head or Tail
Possible combinations are:
LEFT -- RIGHT
Head -- Head
Head -- Tail
Tail -- Head
Tail -- Tail
If you know he definitely has a Head in one of his hands then you can eliminate Tail — Tail – leaving three possible combinations, two of which have Heads in his Left hand. 2 out of 3 = 66%
Nice example Drac.