Samurai Jackson said:
I thought he was asking if mastering two elements that combine like wind and water would maybe allow you to fake a combination element even w/o a bloodline.
No, I was referring to people who are born with the elemental kekkei genkai i.e. if they mastered both nature transformations, would it have any effect of their combination jutsu?
And all elements are able to combine given you have the right DNA. Though, seeing what Onoki and Muu can do, it's possible there's a method to learn to combine them.
Pretty sure people with elemental combination bloodlines can use their required nature transformations instinctively and don't have to learn it. Hence Haku being able to kill his father as an untrained child.
Possible, but to what level? Their bloodline should allow them to instinctively combine the elements but that doesn't necessarily mean they have a fully mastery over it.
And even though they can automatically use the combination, it may not be instinctive for them to use one nature transformation by itself. For example, despite Haku being able to create ice instinctively, he may not immediately be able to cut a leaf in half with wind chakra.
To use a rather poor analogy, it would be like a ninja learning the water-walking exercise before the tree-walking exercise. They wouldn't immediately know how to tree walk despite being able to do something far more difficult though they would learn to do it faster than usual.