well, i would do that by creating a document with very wide page and use the vertical column guides as the vertical separator.
if the goal is to create a pdf with the real pages in there, you will have to look at an advanced imposition tool.
in that case i would create one scribus file for each article and then let the imposition tool.
but, as nermander is writing, i'm really not sure that the pdf format supports arbitrary horizontal layouts. (on the one side i don't see why pdf should not support, on the other side i cannot find any evidence that the pdf can do anything but double sided layouts, on the final side i have no idea what the right search term would be...)