Heroes' Podium will give a bonus to each legendary creature you control, even if gaining control of one causes the "legend rule" to apply. For example, if you control Brimaz, King of Oreskos (a 3/4 legendary creature), and gain control of another one, they'll each be 4/5 when you put one into its owner's graveyard. Then the remaining one will return to being 3/4.