The arrival of Many Treasures Buddha in his Stupa and the image of him and Śākyamuni Buddha sitting side by side are very significant… . The sutra emphasizes the fact that the whole body of Many Treasures Buddha, not just his remains, is present in the Stupa and that his voice emerges from it. But Many Treasures Buddha, we are told, had long ago passed into final nirvana. In this way the whole meaning of nirvana is called into question. And the sitting of the two buddhas side by side violates the assumption that there can only be one buddha in this world at a time. This is one of the ways in which the Lotus Sutra teaches that stories of entering nirvana are teaching devices to get people to be more responsible for their own lives, a theme which is developed most explicitly in the story in chapter 7 of the guide who conjures up a city as a temporary resting place for some travelers who want to quit the journey.
A Buddhist Kaleidoscope; Gene Reeves, The Lotus Sutra as Radically World-affirming, Page 181