This is just logical. If you link with a Black game in the Global Link and you own White and then do a mission, you can visit Black city. This isn't a glitch (except the glitch forest) because both places are in both games. Also, cross-over your sav works because BOTH THE GAMES HAVE THE SAME BASE. GF starts with making one game, and then they copy it and slightly edit the copy to get 2 different games. This isn't weird or anything, if you put a HG sav on a SS game you can catch Lugia the SS way. That's how it is done.
Lol same reply as I posted on UTube <3