The energy of the system does not change. Therefore the energies of each "water" is the same as collective mass and final temperature energy.
m1*Cp*T1 + m2*Cp*T2 = (m1+m2)*Cp*Tf
where
Tf = (m1*T1 + m2*T2)/(m1+m2)
now the collective entropy change of each portion of water is
Δs =...