The easiest way to do this is to make a half darkslide mask. It's lightweight and easy to carry with you all the time. You just remove the regular darkslide and insert the mask to make the photograph, and you can fit two frames on a sheet.
I made mine with an X-acto knife and a straightedge and a spare darkslide. The mask portion should be a little more than half the frame, so you get some border between the two frames, and you need some margin at the handle end to keep the mask straight in the holder and to engage the felt light trap. Insert the full darkslide before cutting and pencil in the margins of the frame to determine where to make the cut.