There are two ways of placing the images. In wall-eyed stereo, the left eye's image is located on the left side of the display, and the right eye's image is on the right. This is the standard method for displaying stereo images in publications as it works well when the display (in this case, the piece of paper) is close to the eyes. It is called wall-eyed because your eyes are directed the same way they would be if looking at a distant wall. In VMD, this method is referred to as ``SideBySide'' stereo.
In cross-eyed stereo, the left eye's image is located on the right side of the display, and the right eye's image is on the left, and hence the name cross-eyed. This is mostly used for distant displays (such as overhead projections) as it is much easier to cross eyes at that range than use the wall-eyed method - you are already looking at the wall. In VMD, this method is referred to as ``CrossEyes'' stereo. This mode is supported by all GPUs.