For shooting once, it is like flipping a 1 coin 2 times one after another, and not 2 coins at once.

Mathematically these cases are equivalent, as long as it is independent occurances, i.e. the result of the second coin flip does not depend on the result of the first coin flip.

What I want to say is, for calculating the probability of at least one of the coins showing tails, it does not matter if you toss two coins at once or one coin twice in a row.

For the two coins at once toss, the probability of at least one showing tails is equivalent to the probability of the roll not being heads/heads. As the probability of a roll of heads/heads is 1/4 (it is one result out of 4 possibilities), the probability being not heads/heads is 3/4 i.e. 75%.

In the same way you can calculate the probability of at least once rolling tails when you toss a coin twice. The only outcome of not having tails at least once is when you roll head twice. The probability of having heads once is 1/2 (0.5), the probability of having heads twice in a row is 1/2 x 1/2, i.e 1/4 or 0.25. As this is the only outcome of not having tails, the probability of having tails at least once is 1-0.25=0.75, i.e. 75% the same as above.

I hope this clears things up on probabilities when more than one roll is involved :-)