This question is given in my textbook:
Consider the radius of both deuterium and tritium to be $2.0 fm$. What is the kinetic energy needed to overcome the coulomb repulsion between the two nuclei? To what temperature must the gas be heated to initiate the reaction? (Hint: Kinetic energy required for one fusion event=average thermal energy available with the interacting particles $=2((3/2)kT)$; $k=$Boltzmann's constant).
Why is the kinetic energy $2((3/2)kT)$ and not $(3/2)kT$?