If both indicate a sexual preference for both sexes, then why do we need to have two separate terms?

Again?

I think....

Bi: attraction to two genders not nessarly male and female

Pan: attraction to all genders

Idk

Bisexuals have a sense of gender; they have different attractions to people depending on their gender. Pansexuals are genderblind and gender doesn't factor into whether they're attracted to someone. They see all humans as humans.

Perfectly said.