Using file, page setup and selecting within the drop down list the size
of paper you need. I am not familiar with A5 card, but its dimensions
can be entered if its length and width are not among the options.
About getting 2 postcards for each sheet either the height or the
length of the page needs to be doubled. Some printers may not handle
excessive pages, so check its capabilities also. A hammer and chisel
may come in handy there.