When I first received the 3x3 double cubes I was eager to figure out how to make them myself, however, I very quickly decided that I couldn't just be satisfied with joining 2 cubes together and the solving challenge that comes with that - I wanted to join 20 cubes together, a few people have inisted that I go further but I am hesitant to go any further at the moment.
Cubes can be joined together by flipping the edge, shaving it down a bit and then glueing it to two corners with the torpedo facing the wrong way, you cxan then insert the corners into one cube and the edge into another. The scary thing about this is that there is no limit really and if I find out that someone has a longer chain of co-joined cubes then I will be very tempted to beat them.
here is the short, although I will upload a longer video soon, maybe at some point I will also have to solve it but I have a feeling that will be difficult.