I read of a change made to the file system / framework / services.jar
But i don't know if it work....
Yes, I'm doing it on my Nexus 4, check this thread:
Another guy is doing it on an Optimus G, he did it first as far as I can tell:
That's incorrect. Android phones can use multiple wireless signals at once (wifi, cell, mobile data, bluetooth, nfc). And they can have multiple network interfaces up at the same time just like any linux box can. It just happens that the network manager turns of mobile data when you turn on wifi, but that's on purpose (to save you battery/cost), and not because the phone isn't capable of it.
Not sure why you say with such authority there's no way around it. If you don't know the answer to something for sure, better don't give a wrong answer.
There's even an app on the play store that can loadbalance a download over wifi and mobile data at the same time: