It's not only rendering the game twice but it's rendering a different version of the game. Lighting, textures, sound are all different. Then take into account AI, physics etc two games being calculated simultaneously.I'm really struggling to understand the "it's running two games at once" explanation for the bad performance: yes, it is very strictly speaking, but it's rendering only half of each instance at any given time. If it were rendering the game twice at full screen somehow, it would make sense to lower the resolution for both. But it isn't, it's making two half renders, the final number of rendered pixels is the same as a normal full-screen game.
From a GPU point of view, it should not be any different from running a split-screen game, at most accounting for the higher average number of assets it needs to keep in RAM for any given environment (but that shouldn't be weighting on the fps all that much). If there is more than that, it just looks like an unnecessary way to fuck up the pipeline.