I think this is a wonderful idea! Also a great incentive for your players to try new tactics and build stronger bonds between their PCs.
Personally as a DM I give PCs free weapon proficiencies if they use the weapon for a full lvl. Consistently taking the -4 non proficiency penalty in combat.
Back on topic though, i don't think this will be game breaking as long as your players are responsible with this gift. However, as DM, if they do start to abuse this you can take it away from them; as is your right.
This may also be a good way to introduce teamwork feats to your group. I find most players don't want to waste one of their precious feat slots on a frankly underwhelming choice, in most circumstances.
