Quote Originally Posted by Troacctid View Post
As far as I'm aware, the CR rules don't include any text that would allow it to work that way, so you would just use normal math. Adding them together gives you 2¼, which then rounds down to 2.
From the dungeon master's guide page 30: "For an NPC with an NPC class, determine
her Challenge Rating as if she had a PC class with one less level.". The orc warrior 1 is written as being CR 1/2 (and not 0), and the orc warrior 10 is CR 9. This clearly shows that reducing CR below 1 creates progressively smaller fractional CR and subsequently increasing CR makes the fraction go closer to 1. This is also clarified in Pathfinder as 1/8->1/6->1/4->1/3->1/2->1.