It has once been said that the multiple species/archetype of dragons would have their own special type of fusion technique that acted out more as a ritual for how it occurs,but once it's done, the two dragons involved in the fusion would merge into one very powerful hybrid of the two and would have it's own skill and transformation tree for them to progress within, Especially for fusions between the two special species of dragons

The Rainbow Dragon of The Crystal Beasts

And the ancient Blood Dragons

However,It would require for the two to have some affinity towards the other along with a drop of their blood or for them both to wear a special type of ring  on their fingers corresponding to elements such as the "Makulan Rings"usually worn by a figure known as the Mandarin

After one of them creates a summoning ritual circle to start it and once all of these requirements are met,both dragons' bodies would glow brightly before the circle would react with the two merging into one being

Recently both Kara Muto and Cassie Blaze have found two Makulan Rings of their own while finding out about the technique and trying it and as for the results of lead to them in this form

Karssie(Kara and Cassie's fusion):*She looks at herself in a bit of surprise when seeing what she became*