Hmm... in Erebus University, Luka, Matthew and Finlay definitely shared one brain cell. They came up with stupid ideas together, broke a fundamental rule together, got punished together.
Although the bigger idiot is either Finlay or Matthew. Finlay stood still watching Luka burned the dorm kitchen, told a terrible pick up line to a witch, and did an incredibly dangerous thing with a werewolf twice among many others. While Matthew recklessly peeped into a warlock's cauldron and faced the consequence, interrupted Finlay's first attempt at flirting by trolling via telepathy, overslept and left Luka stranded in an entirely different continent, got caught smuggling a whole meal for his fellow pledges during Hell Week, and... well, he messed up a lot.
There are two idiots in A Throne of Potion and Mirrors too, but I think Kinalu takes the cake. He simply kept saying things he shouldn't say, doing things he shouldn't do and being in places he shouldn't be. But I haven't finished writing it, so there's still plenty of time for Darga to overthrow him.