the title says everything: there should be a way to make multiple game at once like in real life
for exemple when you start a game, you would have to choose who will work on the game (which also means that we’ll need more employees but for that, I think we’ll need multiple floor)
When Bethesda were making Skyrim they also wanted to cash in on the Fallout 3 trend and created the spin-off Fallout New Vegas. However Bethesda did not make Fallout New Vegas they simply subcontracted it out to Obsidian.
I think the general rule of thumb is one game per studio. But subcontracting for a large fee is a great idea for extra content and long term income. 
like sega and sonic team then (sonic team making… well sonic game while sega making another game)?
Isn’t Sonic a Subsidiary of Sega? The same way Bethesda is a Subsidiary of Zenimax. Thats not the same thing. Thats a parent/child corp. relationship.
A subcontractor relationship would be design studio A hiring design studio B to create a product under strict guidelines and supervision.
We might need a 4th office for that, could be a office with Departments instead of single employees that are able to make multiple games for multiple platforms hint 
This would be useful to adress a major late-game flaw: Portables.
By the time the really REALLY good portable systems hit the market (GS and onwards) your company will generally be tooled for making Large/AAA games. Unfortunately, portable game systems are limited to small and medium games. And trust me, I hate having to ignore the GS because it is the first system to give a major bonus to Strategy games, and my favorite genre to do in game is Strategy.
Having a full-size, AAA-capable team and choosing to develop two medium games at the same time would be a tactical move that might be right for certain types of companies and not others.
Of course, this doesn’t address the other problem with late-game medium titles (Less money for 9.0 hits makes making smaller games a sub-optimal strategy) but that would be fixed if the game were on a ‘rolling industry average’ rating scale, which has been talked about in other threads.