Not long ago, some China company made a 2.5" frame (FUS-X111Pro) that has a very light weight construction, immediately I came up with the idea to stack the two frames up to make it a X8 config drone. I want to build a drone that can carry a full gopro but with more power to have better maneuvering.
The basic idea of this X8 is to combine the two frames using 30mm long M2 screws (screwing them down from the top), with stainless steel hex short standoffs to separate the frames so that the props won't hit the top carbon frame.
Since I try to cut down the cost, I used one set of 1205 4500kv and another set of 1207 5000kv motors that were lying around from previous builds, turned out that they worked very well.
I only did 2 test flights for now, using 4S 850mah I was able to achieve 2.5mins of flight time flying with moderate speed. It should have a better flight time with 1000-1300mah lipo.
I don't have a Gopro at the moment so I am waiting to get a Gopro8 to test it again in the future. Runcam5 is what I used for recording the flight.
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Yes, you'll have more power but your losing efficiency due to the the interaction between the two sets of contra-rotating blades. Its why you don't see this used in aviation much. (see note)
Which is especially true when the motors are spinning at different speeds. This is because the leading blades maybe moving faster or slower than its contra-rotating blade. Even if the motors are digitally controlled to match speed, there will still be some differences. For example, when it comes to the propellers , our props are designed to flex under loading. This propeller flex moves the blades closer / further from each other, creating allot of noise. The noise created is due to the contra-rotating blades pushing through the wake of the 1st propeller and losing energy in the form of noise. Thus I recommend using props that are fairly stiff to minimize the amount the props flex.
Beyond this, you'll most like be able to fly just fine but not quite as long with half the setup. I expect though that the flight will be pretty loud as Ducts tend to amplify this contra-rotating noise considerably.
Note: In the 90's Boeing tried to create a passenger plane (7J7) using counter rotating props. https://en.wikipedia.org/wiki/Boeing_7J7 Sure they were more efficient than a regular jet (60% more) but later found them to be really loud and not quite worth the effort due to the added mechanical complexity. Keep in mind that this was also at a time when the price of oil dropped considerably and aircraft subsidies increased. Thus making the concept look less financially attractive to the airlines.
I re-read that your using two different sets of motors with unlike KV ratings. You'll find that the higher KV set will draw more power and possibly get pretty warm. During high throttle conditions, the lower KV set may not be able to match the speed of the first as its just not physically possible. I expect that if all the motors were the same, that your flight times would improve by a minute or more.