When I chose to grab the Holga 135 from amidst all my cameras, the way I take pictures changes the possibility of shooting double exposures.
However, this one is my favourite. The steps that you have to follow are the ones below:
1. Take a picture wherever you want but in a place where the sky is the half part of your frame (the upper part).
2. Turn your camera.
3. Take the exact same photo that you did before with your camera turn it upside-down and with the sky at the other half of your frame (now in the part below).
Your photographs will be doubles, will be somewhat special and more sharp because the sky will play the leading part at your composition.
I encourage you to give it a try!