The thinking behind splitting DLC into two different modules, without duplication, began around 2 years ago.
Pathfinder 2 also splits the data into two modules - if appropriate, one for the GM and one for the player, with little/no duplication across the two. There's currently one exception to this, the Lost Omens World Guide - the first non-core setting guide released before the shift in design methodology, and that will be reworked at some point.
For Pathfinder 2 the split is made based on GM only information - i.e. information that the GM may not want the players to see as standard. The whole of the Core Rules is deliberately made as a player module, as that's expected to be a player resource (there's no "player handbook" and "DM guide" type rules split in the Pathfinder Core Rules) - you could argue that some magic items might be GM only initially, but that's a minor point and it could equally be argued that most of the magic items should be accessible to players.
For example: The Lost Omens Character Guide has most of the data available to players (as you would expect for a character based product), but a small GM module has been made that includes details on organization adventure hooks NPC gallery and Theme Templates; all information that is GM specific and the GM may not want the players to see. If the GM does want the players to see this data, they can give them access to this module without causing any duplication. By default the GM will load both the player and GM modules for each product they want access to.
The split only really effects the reference manual - having to jump between two reference manuals to get the full product information. Other data is displayed in the campaign data lists and grouped lists - which is where duplicates could become problematic; one of the reasons for splitting the modules without duplication (the other main one being data record links accessible to players). So, it's a trade off - the inconvenience of having to flip between two reference manuals, to the advantages of no duplication and valid data record links.