There are like 3 teams from Namco working on Smash Bros. And I'm assuming Nintendo's second party studios like Monolith Soft and Hal Laboratory are helping out as well. Even in that Nintendo direct they confirmed that they wanted to accelerate the process of making Smash Bros. considering it's known to have really long development cycles- at least, Brawl did. Melee was rushed for launch for the Gamecube. Sakurai had no holidays and even on weekends he had to work on Melee for a straight year. With Brawl, he had no team. And eventually, Game Arts took over the development (at least most of it) while Hal Labs just provided the assets (character models, Melee "engine", etc)
For this Smash Bros, it does seem like one team from Namco is handling the graphical assets and another the gameplay mechanics while Sora Ltd also works on the latter. They even had a prototype of Smash Bros. 4 working last year. So, I'm guessing development is going very very smoothly with all the help. I do think the 3DS version may come out first before the Wii U game. Since the two are bound to connect to each other, I'm thinking the 3DS version will have some RPG elements when it comes to customizing your characters so then you can take them to the "stadium" that is the Wii U game. Look at it as Pokemon Red/Blue connecting to Pokemon Stadium. The 3rd team working on Smash Bros. are the folks who make the Tales of series of video games. I'm going to hazard a guess that they're handling the 3DS version for the most part.
Of course, these are just my thoughts and series from what I have observed over the past year. Work started probably in May. But that wouldn't make sense because Sakurai said around that same time that the prototype is looking excellent, made by Bamco. Maybe Nintendo enlisted them far ahead of time without Sakurai's knowing prior? Eh, who knows. All I know is that we'll be getting Smash Bros. 4 sooner rather than later. I'd probably say the latest being the Fall of 2014. Nintendo has other titles and have recently been spacing out their releases to avoid cannibalization of their software. And because Nintendo is in an urgent position with how the Wii U is doing, I have no doubt in my mind they're pushing Sakurai and the teams to get this game out asap. Just, thankfully this time, Sakurai doesn't have to work with a small team.