in the symmetry plane of this system , each point is at the same distance from the two threads
let the distance be r
then the eletric field due to both the threads = λ/2πrε0
but they will be inclined at 2θ where sinθ=l/2r
so the net electric field will be along the plane=
2cosθ(λ/2πrε0 )
differentiate it to get maximum