transmission coefficient as a function of energy and spacing for two delta potentials at delta(x+a) and delta(x-a)

T[k_,a_] = 1/(1 + 4 k^{-2} (Cos[k a] + 1/k Sin[2 k a])^2)

Plot3D[T[k,a],{k,0,5},{a,0,2}]