As of now, the manga has provided us with 8 different dual combinations out of 10, 2 are seemingly yet to be shown, but I'm here to challenge that notion. It's very unlikely that after so many years, we are still yet to be shown the remaining 2 combinations, so I came to a conclusion that most likely we have already seen all 10 by now.
The supplementary novels are supposedly canon, as they were placed on a timeline by Shounen Jump, but people still argue over this. By the notion that Gaara Hiden is canon, so is Steel Release then, as it appears in said novel. If we move on and accept that, now we have 9 out of 10 dual combined nature transformations, just 1 left.
What else do we have that we could possibly use from outside the manga...
- Crystal Release
- Mud Release
- Dark Release
- Swift Release
- Typhoon Release
So let's make a small analysis. Typhoon Release doesn't seem to be a dual combination nature transformations, rather it's the most similar to Blaze Release. As Blaze Release is a stronger kekkei genkai version of Fire Release, Typhoon Release is a stronger kekkei genkai version of Wind Release, most likely. For Mud Release, it's the only combined nature known to be made of the same basic natures as another one, Wood Release, or is it? Now, folks might be scratching their heads and thinking that this is in no way or shape canon, but consider the following notion: What if the same basic natures combined can result in more than just 1 combined nature? It may seem far-fetched, but no contrary evidence has been shown so far and if the novels are canon, we must accept this as a thing.
In fact, we might have had this appear in the manga already. Mei uses Lava Release, but according to 4th databook, her Lava Release is unique to her alone. Even though Mei's nature is pronounced the same and called the same thing in English (Lava Style) it's written differently, 溶遁 compared to 熔遁. So what if, even though both Lava Releases use the same basic natures, are actually 2 different Kekkei Genkai? Mud Release compared to Wood Release and Mei's Lava Release compared to the others' Lava Release may very well have the same kind of relation, both possibly showing such a phenomenon exists within the series.
Now, Swift Release and Dark Release are for all intends and purposes not even slightly canon, as the movie they appeared in isn't part of any officially released timeline, nor have its specific characters and events been referenced in canon.
That leaves us with Crystal Release. It's quite an unique one compared to Swift Release and Dark Release. Despite its appearance in a filler and the short filler being the nature's only appearance so far in the series, we were given far too many techniques for me to think that Studio Pierrot came up with it all on their own in their free time. I'm inclined to believe that Kishimoto might have had influence on this nature's existence, solely for the reason that it and so many techniques for it just seems out of Studio Pierrot's scope of reach for me. Therefore just for the purpose of this topic, let's assume that Crystal Release is canon and is the 10th dual combination nature.
Let's now take a look at the combined natures' compositions themselves. (yes, I'm sure you saw it comin') If Rasa's 4th databook profile is to be believed (as the databook is known for errors and omissions and who knows, maybe even erroneous inclusions, mistakes in general...) then Magnet Release is Earth+Wind. Well, I and many others within the fandom are not entirely convinced, as the very source is questionable. Another one is Explosion Release, validity of which depends on the premise that Magnet Release indeed is Earth+Wind and also an obscure statement from Sasuke's fight with Deidara. Allow me to be skeptical in this case and to provide my own view on them.
First, let's list all the combinations that are pretty much confirmed before we move to the rest:
- Ice Release = Water+Wind
- Wood Release = Earth+Water
- Lava Release = Fire+Earth
- Storm Release = Lightning+Water
- Boil Release = Water+Fire
- Scorch Release = Fire+Wind (some still want to question this.... seriously?...)
Unconfirmed, my take:
- Explosion Release = Fire+Lightning (the clay itself was Earth Release)
- Magnet Release = Lightning+Wind
- Crystal Release = Earth+Wind
- Steel Release = Earth+Lightning
Now you might ask 'why do you think your speculation is any better than the other person's out there?' Glad you asked. My take is the only take that provides us with all 10 combinations, any change and it's not possible to get all 10. Only by acknowledging Steel Release as canon and assuming Crystal Release to also be canon, do we get 10 likely combinations and only by questioning the 'information' provided for Magnet Release by Rasa's profile in 4th databook and therefore consequently assumed Explosion Release, can we fit all the natures nicely together like I have.
Thank you for reading <3