Well...
You can only train 3 guys at a time. And probably normally only 2 guys well, given bb loves to stuff up training minutes
And it would probably take 7-8 season's to train them.
And since you can only train your trainees the same way, you either end up with two bags who can't defend inside, or two small who can't defend outside. Which is useless without their opposite player.
And since not many of these players exist, you'd be hard pressed to nab one off the market.
It's an interesting idea, but would require a lot of effort and coordination to work, and you'd probably need to cooperate with another manage to train up the opposite player to your guy, and then trade. And even then, you only end up with 2 players like this, not a teamful.