But by the time the A team both developed Platinum and HG/SS at the samr time, team B was busy to develop the 5th generation. And I remember they said they started with BW early 2008. That are also only 2 years of development while XY started 2010 with XY taking 3 years by both teams. Yeah XY needed all the time for the big change to a 3D overworld and the massiv amount of polygon models for the pokemon, but still.
Most likely B team developed Platinum and HGSS, while A team started BW around 2008. Team A does the main projects, while Team B does the side projects.
What you are forgetting is that B2W2 and XY, started developement at the same time, so while Masuda and Team A were developing XY, Takao Unno and Team B were developing B2W2. Which means XY only used the full staff during the second half of the production period. During the first half of XY development, Team B was making BW's sequels...
While ORAS used the full team during the entire production period, since no other game was developed at that time.
And XY being the FIRST 3DS project they made, they needed a lot of time just to change all the 700 Pokemon from Sprites, to 3D polygons, this alone would have required months, I guess. Masuda said in an interview, that for ORAS, they had made a lot of the work already in XY (meaning the 3D models, the PSS System, Super Training, Amie, Battle Maison, etc). All are in ORAS, but were made before ORAS production started, so as you can see, XY saved ORAS a lot of work. And it could be argued that ORAS has been in production for more than a year, if we count all those things done during XY's production time.
They were making XY, but in reality, they were also making ORAS, before they really started to produce it, if you know what I am saying.