Initially this seems like a good idea, but what you're going to do is cause turbulence inside the plenum. If you fire air in from both ends, sure you may achieve higher pressure, but the air will be turbulent and result in minor losses. After all, this is why porting / flowing is a thing
DISCLAIMER: This is for indication only, this could be a very bad idea, I have no experience in modifying intakes, do not attempt this unless you're bat**** insane.
A small amendment to your idea would be to modify a stock plenum that fed the left 3 cylinders from the left and the right 3 from the right side (forgive the crude photoshop)..
dual-intake-plenum-flow.png
The gold band between the red and blue arrows would have to be cut out with an angle grinder and a aluminium / steel cylinder inserted and welded in to stop airflow between each bank of three.
There are a lot more things to consider here too such as cylinder firing order, possibly requiring 1 dual output or 2 single output intercoolers etc...
As for gains, I think the only situation this would really help would be for those going twin aftermarket big turbochargers, the idea does give food for thought though, with this turbos could be located in alternative positions etc..