Why P value of a 2 tailed test is multiplied by 2 [=2 X P(Z>tcal)? I am looking for a answer which may explain the underlying reason except the answer 'because it is a two tailed test'. Why we also consider the opposite signed value of test statistic in two tailed test to calculate P value?
why the P value of a two tailed test is multiplied by 2?
Not all two-tailed tests have the property that a p-value of a two-tailed test should be double the p-value of a one-tailed test.
However, tests where
i. being in either of the tails are mutually exclusive events*, and
ii. the distribution of the test statistic is symmetric
will have the property that to compute the p-value of a two-tailed test you double the smaller p-value of the two one-tailed tests. This follows because a p-value is the probability of a test statistic at least as extreme as the one you observe under the null hypothesis; the two tailed test considers alternatives in either direction so "at least as extreme" can be in either tail; given the two conditions above, you get that doubling of the one-tailed p-value (which only considers one of the tails).
* consider the two one-tailed Kolmogorov-Smirnov tests compared to the two-tailed version to see a case where being in either tail are not mutually exclusive events.
For the asymmetric case, some discussion here may be relevant.