So I just played a game with 
on Hexakill and got an S, which is a big deal for me because i already had 1 Mastery 6 token... but i didn't get a second mastery token after the game.
Do you not get Mastery 6 and 7 tokens in the featured game modes? i'm confused.
i dont think you do, just chest, keys and masteries 1-5
Only get tokens from SR normal/ranked
"Champion Mastery Levels 6 & 7
Earning an S- or above on champions with level 5 mastery will net you Mastery tokens. Combine these Mastery tokens with Hextech loot in your Hextech Inventory to reach Mastery levels 6 & 7! Here are the details for each rank:
Earn Mastery 6 tokens for S-, S, and S+ games with champions that are Mastery Level 5. Combine two Mastery 6 tokens with that champion's Hextech shard, permanent, or 500 champion essence to unlock Mastery Level 6.
Earn Mastery 7 tokens for S and S+ games with champions that are Mastery Level 6. Combine three Mastery 7 tokens with that champion's Hextech shard, permanent, or 600 champion essence to unlock Mastery Level 7.
Mastery tokens, champion shards, and champion permanents used in unlocking Mastery must all be champion-specific, so if you want to hit Mastery Level 6 with Quinn, you'll need to earn Mastery 6 tokens by hitting S rank with her.
You can only earn Mastery 6 and 7 tokens in Summoner's Rift normal and ranked games."
https://support.riotgames.com/hc/en-us/articles/204211284-Champion-Mastery-Guide