Bunuel wrote:

The first PDA goes to Derek and the second to Lena - 1/60*1/59;

The first PDA goes to Lena and the second to Derek - 1/60*1/59;

Thus, the overall probability that both PDA will go to the couple is 2*1/60*1/59=1/1770.

Hope it's clear.

Just for understanding it better, is it right to think in following way:

In 1st case , till the time Derek receives the first PDA every of the 60 participants including Lena have the equal probability of 1/60 of receiving the 1st PDA and after its confirmed that Derek does receive the 1st PDA, the remaining all participants including Lena will have 1/59 probability of receiving the 2nd PDA.

^^^ thinking above on the lines of 'row' concept.