I did some experimenting with the 2 Nadroji bonus's, and it only uses 1. the other 100% comes from someplace else, either from being on a challenge map or very high floors, or maybe even random. when I was on a map with 0% total enemy rarity. I used both of the bonuses. I even tried doing hat/chest and chest/hat when equipping, and only got 100%, so the other 100% is from something/someplace else. I think the reason there are 2 bonus is for different builds that need the bonus either on the chest or hat. I have had 100% total enemy rarity without the Nadroji Bonus, so I think the max is 200%, but 100% is from someplace, and the other from Nadroji.
as far as the bonus goes, there is a post that states that the rarity bonus increases the chance of the monsters spawning as Magic or Rare tier monsters. if you are on a pack size map, then any monster that spawned as Magic/Rare rarity will come in larger packs, just like the Normal monsters. higher tier monsters mean better loot and more experience, but more danger. having a Rare 150% pack size furies or malums landing on you just is not a fun time, exciting, but not fun. unless you don't die right away, then it is exciting and fun!