Two particles are projected from a point at the same instant with velocities whose horizontal and vertical components areu1,v1 and u2,v2 respectively. The interval between their passing through the other common point of their path is
x=u1t=u2(t+Δt)−12g(t+Δt)2,
y=v1t−12gt2=v2(t+Δt)−12g(t+Δt)2
⇒Δt=2(v2u1−v1u2)g(u1+u2)