50%
Assuming that the coins are fair and each have a 50% chance of
head or tails there are 4 equally likely combinations that can occur:
first coin is a head and second is a head, first a head and second a
tail, first a tail and second a head, first a tail and second a tail.
Each is as likely, 25%. The four choices are HH, HT, TH, TT, and you
can see that two of them (HT, TH) fulfill having one head and one tail,
giving you 25% +25% making is 50%.
