Should I look for a player with similar stats so I can train them both at the same things?
That would be ideal.
I managed to find a HOF 20yo that was within 2 on every stat to train alongside the 18yo HOFer I drafted. Fast forward 3 seasons and they are still very similar but differ slightly from training speed differences due to height and age. Very pleased with how it worked out thus far.
I think it works great for a while but as they get to demanding higher and higher salaries you'll have to reconstruct the team or pick one to keep and one to profit from.
Good Luck.