妙法蓮華經玄贊 8.511

8.511

贊曰：此坐也。問：何故二佛同坐一座？論云「示現化佛、非化佛、法佛、報佛等，皆為成大事故」。即為成一大因緣，又表三乘同坐一乘之床，眾生亦同一乘座故。

The commentary says: This is taking the seat. Question: Why do the two buddhas sit together on one seat? The Treatise says: "It demonstrates that emanation buddhas, non-emanation buddhas, Dharma buddhas, and reward buddhas all accomplish the one great matter." That is, to accomplish the one great cause; furthermore, it signifies that the three vehicles sit together on the couch of the Single Vehicle, and sentient beings also share the seat of the Single Vehicle.