Pokemon trainers enjoying the Sinnoh region remakes in Pokemon: Brilliant Diamond and Shining Pearl are likely well underway towards completing their Pokedexes, but the problem of registering exclusive Pokemon from the opposite game always persists.
Dialga and Palkia, two thirds of Sinnoh's Creation Trio alongside Giratina, are made exclusive to their respective titles in most cases, with Dialga residing in Brilliant Diamond and Palkia in Shining Pearl. Trainers hoping to complete their Pokedexes, at least as far as registering spotted Pokemon, will need to eventually spot the opposite mascot within their own game. Fortunately, this is possible in Brilliant Diamond and Shining Pearl.
Where to spot the opposite legendary Pokemon regardless of BDSP version
In order to register the opposite mascot Pokemon in their Pokedex regardless of version, trainers will first want to catch the mascot legendary tied to their version.
That means making it to the top of Mt. Coronet and defeating Team Galactic at the summit. At the end of the encounter, trainers will have the chance to catch either Dialga or Palkia based on the version of their game.
Once that's completed, Pokemon trainers will want to head to Celestic Town. North of the cave is a large building which houses an elderly individual wearing a purple outfit. After capturing their mascot legendary, speaking to the old man will initiate dialogue pertaining to an ancient text that possesses information on the Creation Trio. As players progress through the conversation, they'll eventually be shown the legendary Pokemon from the other version.
Although many Pokemon games required trainers to actually see the Pokemon in the wild to register it in their Pokedex, this isn't the case for Brilliant Diamond and Shining Pearl.
Seeing the legendary within this ancient book will register the Pokemon as seen within a trainer's Pokedex, and they'll be one step closer to completion. As for acquiring the legendary in question, that's another story entirely and will take much more effort than simply speaking to an NPC.