In response to all of the negativity towards the new design, I just want to clarify some things and try to explain some of the design choices. I've heard people call this a cheap photoshop work, lazy and uninspired and that they did it so much better with the /Dark and the /Metal dual type cards in days yonder. You are welcome to feel that way, the old dual type cards do look really cool with the combined symbol and the faded mixed colors.
I feel like this execution is perfect. For one, its clear. It shows the most clear terms, without the need of an extra text box to explain what the card means, that it is both fire and water type. This is useful for the younger audience, and even older players that got into the game after 2007.
Now, the diagonal line was done in a very specific way. Lets treat the cards in 2 halves, a top and bottom half. For the top half, the diagonal line crosses directly in between the overlapped type symbol, separating fire and water in a subtle way that doesn't impact the artwork too much. For the bottom half, the diagonal line divides the card in to equal halves, with the top-left portion of the bottom of the card being fire, and the bottom-right portion being for water. This is also reflected in the artwork with the fire and water splash effects, and then in the top portion of the card, both the fire and water melt into steam, which is Volcanion's main mechanic. If you treat this diagonal line as such, the reasons for why it is oriented, and you begin to understand why it is actually the perfect dividing line, again to make it as clear as possible that it is a dual type Pokemon.
On to the overlap with the border which I have to say, whatever. For EX's everything overlaps the border already, so having the energy symbol do it now is just allowing for more room. If they make dual-type cards that aren't EX's, where there is more room for the HP and type symbol without having to place around the artwork (or more commonly, the artwork drawn around the text) we may or may not see that overlap with the border.
Lastly, the old combined type symbols were cool looking, but there was only 16 of them at a time to make and worry about, since every type was either combined with dark or metal. So they only had to make type symbols that combined Grass/Dark, Fire/Dark, Water/Dark etc... The new dual type mechanic removes that restriction on the /Dark and /Metal, in this case we see a Fire/Water type. This opens up the possibility of Psychic/Fairy, Water/Grass, Water/Electric, Dragon/Psychic etc. The sheer amount of different combinations makes combining the symbols way more work than necessary, only to end up confusing a lot of people if they see a fire symbol in a blue circle or a Psychic symbol in a pink circle. As well as the confusion that would arise from combining the different colors on the main body of the card. The subtlety of those combinations make it really difficult to understand on a quick glance, what the card actually is trying to accomplish with its dual types, and as in the older cards, actually required a text box to explain that the card was both Fire and Dark, or Water and Metal etc.
Is it easy for you to understand with a blended design? Sure, I bet you had no problems what so ever, but that isn't the case for many players, and Pokemon as a ever-growing game that has to appeal to all ages, and be clear enough for new players to pick up right away, these design choices seem perfect for doing that.