A single coin toss results in either a head or a tail, but not both simultaneously. Therefore, the probability of getting both a head and a tail in one toss is 0.
When a single coin is tossed, the sample space consists of only two outcomes: getting a head or getting a tail. Getting both a head and a tail simultaneously in a single toss is an impossible event. The probability of an impossible event is always defined as 0. The probability is 0.