Let the charge Q is divided into two parts of q and Q-q
let parts are sepereted at a distance of d
so repulsion force
F= k q(Q-q)/d^2
here Q and d are constant so for maximum force differentiate the force w.r.t q
dF/dq = k/d^2 [Q -2q]
for maximum force dF/dq=0
Q-2q=0
Q/q =2
So the charges are Q/2 and Q/2