Bundling two completely independent quadcopters together to form an octocopter, the following is a test flight (sorry about the occasional focus lost, kid helped shooting the video)
I've been wanting to try this for so long and since it's kinda drizzling out there today, I finally got time to test this out. Here's what I did, took two of my beater racing drones, flip one of them by changing the motor order and FC orientation and then use zip ties to tie them together. Voila! I got a X8 octocopter! This is just a proof of concept, but imagine what you can do with this configuration -- if you configure one of the drone as 3D mode you can slow your quad down without air brake, the "brake quad" can use smaller motors and work as a boost or brake. The extra redundancy here should also help with those cinelifters which carries expensive cameras, i.e. if one system fails, the other can at least help the whole system to land.
This is a very easy experiment to try at home. Looking forward to seeing newer experiments. If you have one too, I would love to hear!