Earning 2 S-grades in 1 game unlocks 1 chest only, but locks out both champions for the season.
Last night I played a normal game with 2 other friends. Both myself and one of my friend got an S-grade on a champion (TF and Shen) we have yet to get an S grade on since hextech crafting came out. At the end of the game, we only got 1 chest, but now both TF for me and Shen for my friend has been locked out for the season as available for unlocking chests. Essentially, we lost a chest because we used twice as many champions needed for that 1 chest.
Either we should be able to get multiple chests per game, or only 1 champion should be locked out of being eligible for hextech crafting. Otherwise this encourages people to purposely not get an S to on specific champions in game if another person is doing well in order not to waste a chance.