Yes the probability of tossing a coin will always be 1/2 but that is because the sample space only consists of a head and a tail. When you toss two coins or toss one coin twice you increase the sample space you can now get either hh ht th or tt so thr probability of two heads is less (1/4) this increases exponentially if you were to toss 10 coins... It's not increasing the probability its increasing the sample space