Do I understand correctly that the item "Bonus code for Original Sin - Enhanced Edition for GOG" in Divinity: Original Sin - Enhanced Edition Collector's Edition means that when I purchase this version I will get two keys?

When you purchase it Divinity: Original Sin - Enhanced Edition is added to your library and you get one extra key. To access it you go to the GOG website --> your library --> click on D:OS-EE --> More --> Serial Keys --> and link appears.