Technically, yes. But in fact, it doesn't really improve the resolution. When the 3D is on, there is one 240x400 screen for each eye. So the extra pixels are only useful for 3D, not for a better resolution. Actually, just imagine the aspect ratio if the screen was really 800x240...
No, that's not how the 3D works.
Even though the total pixels on screen are 800 across, In 2D mode, the effective resolution is 400x240 because the image displayed is stretched over the sub-pixels 1:2. However in 3D mode, that same image is reconfigured by the framebuffer to only display on every other 400 sub-pixels 1:1, leaving the other 400 pixels to the 2nd framebuffer which displays a different image of the same scene at a different angle. Because of this, the effective resolution in 3D mode is 800x240.
This is because each image has its own detail/information that the other image does not display in its 400 pixels. For the 3D effect to work, one image is displayed to its respective eye, the parallax barrier blocks the other eye from seeing the image displayed to the opposite eye. Each eye gets shown the same scene at slightly different angles showing different detail. It doesn't matter how different one image is from the other, as long as the image displays aspects of the scene not shown in the other image, more detail is perceived when the brain merges these two images together to get the 3D effect. This is why objects/environments seem "rounded" while each individual image is just a flat, 2D, 400x240 shot. You're seeing both sides of an object at the same time. Your brain perceives both images at the same time and merges both aspects of the images into one 3D scene, displaying what both images have to offer, 800x240 resolution worth of detail. 2D mode is just an automated version of simply closing one eye with the 3D effect on, you only see one of two images when closing an eye.
The notion on this thread that 3D is 400x240 is physically impossible as that would suggest that either you are not seeing a 3D image in 3D mode at all as that would require a 2nd, slightly angled image to get the "rounded" effect, or that your eyes/brain somehow cancels out the 2nd image's pixels/detail leaving you with only the first images 400 pixels/detail but still a providing 3D effect, which is not only impossible but hardly makes sense. You have to see both images to get the 3D effect, the viewing angles are wider in 3D mode and more is being shown, you are seeing an 800 pixel wide image in 3D mode or else the 3D effect cannot happen. This should be common sense by now.