I like this suggestion. And I like it for only one reason: It reduces the need for tanking in order to train.
As it is now, if you train one 18 yo player, you are forced to train him 1vs1. And if you train 1vs1, you are forced to have all very young trainees (18/19 maybe 20 y.o.) because there is little use in 1vs1 training for older players. And if you train very young players in every game, you cannot compete. So you are forced to tank.
With this suggestion, you could train a couple of 18/19 y.o players on Wednesday and train older players in league games, which would allow you to compete.
This is an important thing, because tanking is a big cancer for the game.