Imposing some degree of control can increase the probability that your doubles will turn out good.
Double exposure is one the most common techniques that lomographers enjoy. The idea of two images from two places, from two different points in time, in a single frame is truly fascinating, especially if the images seem to complement or relate to each other in a serendipitous way. However, if we completely rely on luck, and trial and error, then the chances of achieving great doubles become less probable. So, if you would ask me, I think that imposing minimal control in terms of aligning the frames and choosing what to shoot increases that probability.
On Aligning Frames
Aligning frames is not a problem for cameras which have a Multiple Exposure switch such as the LC-A+, Fisheye, and La Sardina, which you intend to use. However, if you plan to shoot an entire roll, rewind it and load it to another camera (such as what we do in film swap) then mark the first two frames with a permanent marker to be used as guide when you load it to another camera. This can assure that the frames are somehow aligned. Of, course others actually prefer that the frames are misaligned for more bizarre results, but as for my experience, misaligned frames are very difficult to scan for laboratories, and sometimes they crop the photos the way you would not want it to be.
*On Choosing Subjects to Shoot *
Another form of control would be in terms of what to shoot. This is merely based on my experience in doing doubles, as well as appreciating others’ doubles. I noticed that for the most part, good doubles are those which demonstrate the basic foreground/background principle. When I shoot two foregrounds or two background, they do not normally work. Imposing some control in your choices, at least for one of the two layers of images, can increase the chance of achieving good doubles. Here are some background suggestions for you to try:
Cloud formations are great background subjects for doubles, especially the once that are a bit dark and shows some silver lining.
Trees braches and leaves are also great.
Flowers, especially when there are a lot of them.
Dark colored or textured walls work well too.
Shadows or shaded areas ascertain that there is enough room for another image.
Light colored texts on dark background are a joy as well.
Light paintings or computer generated lighting or colored slide presentations for Revolog imitations.
Dark colored patterns and textures.
Umbrellas under the sun are awesome too.
Silhouettes are a must.
There are other ways to improve your doubles technique, but if you try any of these suggested backgrounds and pair them with as distinguishable foreground, half the battle is almost won.