The angel made by relative motion of projectile 1 with respect to 2 with X- axis (horizontal) is:

projectile 1 initial velocity=u1;  velocity at any point =v1=u1xi^+(u1y-gt)j^  projectile 2 initial velocity=u2;  velocity at any point =v2=i u2x+j (u2y-gt)  relative velocity of projectile 1 with respect to 2 =v1-v2=i [u1x- u2x] +j [(u1y-gt)-(u2y-gt)] =i u12x+j u12y   angel made by relative motion of projectile 1 with respect to θ, then tanθ=u12yu12x