Which two Reubenites joined Korah's rebellion against Moses?

Answer: Dathan and Abiram

Korah son of Izhar was joined by the Reubenites Dathan and Abiram, sons of Eliab, and On son of Peleth, with 250 leaders.

“Now Korah son of Izhar, the son of Kohath son of Levi, along with some Reubenites—Dathan and Abiram, sons of Eliab, and On son of Peleth—conducted a rebellion against Moses, along with 250 men of Israel renowned as leaders of the congregation and representatives in the assembly.”— Numbers 16:1-2 (BSB)