It would be awesome if a game was titled "alternating dragons". I am not 100% sure but I suspect they wanted to have 2 player mode but couldn't due to time restrictions for development.Nah. It was hardware restrictions.
The hardest one? Tough question. But I'd say The Combatribes (arcade) is among them. It's also by Technos. Yes, the same creators of Double Dragon and River City Ransom.It's possible. It just takes hours and hours of practice. I watch people on Twitch practice arcade games to 1CC them all the time.
It's also a beat 'em up. There are 6 levels total, and you start the game with 150 hit points. Thing is, you carry on that HP through the ENTIRE game. There are no means to regenerate it, and clearing a level doesn't heal you either - you begin the next one with the same amount you had after finishing the last. Enemies are pretty aggressive and it's really hard to avoid getting countered when you're hitting them, and nearly impossible against bosses. Plus, on the last level you have to beat ALL previous bosses in a boss rush. You only have 500 seconds to do that, and they have quite a bit of HP - making it also a battle against time. Heck, the final boss at the end is brutal enough...
So yeah, It's a harsh game. Even for arcade standards. There's a video of someone beating it on YouTube, but they save-state'd their way through (the final level was cleared with only 11 seconds remaining, btw). I don't think it's humanly possible to beat this with 1 coin legit.