I am going to walk through changes that are required to make Nav2 recognize two robots, and - unlike in official tutorials - I'll do my best to explain what and why is done. Strictly speaking, walking through code file-by-file is probably not the best strategy here, but an alternative is to focus on changes, and for each change I make, walk through all files, as it takes changes in multiple places to support each of few changes... Which is too long.
Instead, I am going to show you what is changed in a particular file, and when we get to the next file, I will remind you that "this place works together with changes we made earlier in that other file".