The first link:
Heads or tails
this side or that side; this thing or that; - a phrase used in throwing a coin to decide a choice, question, or stake, head being the side of the coin bearing the effigy or principal figure (or, in case there is no head or face on either side, that side which has the date on it), and tail the other side.
The second link:
When a coin is tossed, we know there are two possible outcomes: heads or tails. Don't ask why we use the plural form. Anyway, either outcome is equally likely. Over many tosses, the number of heads and the number of tails will be about the same.