Some time ago, I knew that in 2HG , attacking required to specify who am I attacking which creature (and that's how combat damage will be dealt to each opponent) however I was unable to find that rule recently, and in addition I was informed that combat damage to which opponent is determined in the damage step in 2HG..

In 2HG games, you do not attack one specific opponent or the other–instead, you attack the opposing team as a whole. Then as combat damage is assigned, if any of your creatures would assign damage to the defending team, you choose which specific opponent on that team that creature will deal its combat damage to.