It does exist. Even the GBATemp moderator announced it a while back that it exists.
No, it "exists" and was "confirmed". As of now, there is absolutely no legitimate proof that the DSTwo+ does or ever will exist. It was "confirmed" in January to be released around the end of the month. February rolls around and they say it might be out by around the end of that month. March rolls around, and we stop hearing much of anything. April came and went without any kind of communication unless you count that one member of the team who keeps posting on the forum about it.
Plus, what do we actually know about this cart? Nothing. We got a maybe release date and no other information in regards to available features. We don't know what firmwares it will work on, what it will do, what the price will be, nothing. To top it off, despite apparently confirming it, the Supercard team has made no effort to update their website with the news of this cart, which to me is a major red flag. Why, as the team, would you not be making an effort to get your cart known in a market already covered in Gateway, Sky, and their respective clones? In the long run, I believe Supercard was simply trying to ride the hype that was generated with the DSTwo+ nonsense that was circulating on the forum and thought that maybe they could turn out a cart to make some money. After all, the confirmation we got was super vague at best and did not give the impression that they were as close to done as they were trying to say they were, if they had even started in on the project at all.
Nothing about this seems right. Either this is going to be a sloppy release that comes out of nowhere once it actually happens, leaving people disappointed as it does nothing different, or it will continue to be that cart that people keep wondering about until eventually people respond to questions about it with nothing more than "lol".