If you split their training you will probably waste money.
Right now at 20 years old their value on the transfer market is driven mainly by their potential. And with that I don't mean potential like All-Star or Superstar, but what they could become if trained right for a few more seasons.
So training one of them heavily and selling the other will be the more economical solution.
If you split training, you will have two rather mediocre players which will not return you a lot on the market and will not help your team either.
So better get some cash for one of them now and have a great starting five or backup player later.