There are several possibilities of arranging TV screens in order to create video walls: from two rows and two columns of screens (also called 2x2), long TV rows (4x1, for instance) to mosaic. Whenever you deal with video walls, you need to take into account the total resolution created.
For a single Full HD screen (1920 x 1080px), you only need to create your campaign and adjust the layout. In the case of a video wall, you have to sum up the width and height of all screens and create a custom resolution layout with the two sums.
See below several video wall setting examples and their custom resolutions:
For a mosaic video wall, you need to place the images on the campaign divisions so they match their correspondent physical position. So, if you have a mosaic with two screens on portrait and two on landscape mode, you will need to place two images in portrait mode and the other two on landscape (confer image below). When you see it in your OnSign TV account, the video will be completely distorted, but with the arrangement of the screen positions on the wall, the image or video pieces match: