Is it better to train 2 112% players at once to upgrade them to 120 or one by one ?

[Toxcatl]

It would better to boost one player ( in a key position such as dc or gk or st) to 120% , he would be very helpful against stronger teams.

All your players may be 120% at the end of the season if you farm greenies and do the right training.

[PgTuY]

All your players may be 120% at the end of the season if you farm greenies and do the right training.

tip for this notorious 'right training', Toxactl ?

[Toxcatl]

tip for this notorious 'right training', Toxactl ?

Sure.
Do not waste condition on over-resting. Buy only FT-players. Don't keep players older 21 except those who have really good performance. Don't do 2x bonus training, because you are wasting videos that can be spent on green packs.
Oh yeah, and use High pressing and m2m marking only in hard and important games because they suck condition.

And it's Toxcatl =]