Looking at the Wikipedia page, I see that the equation of a straight line corresponding to a given given r, θ pair is
r = x cosθ + y sinθ
Thus, if I understand, given two pairs r1, θ1 and r2, θ2, to find the intersection you must solve for the unknowns x,y the following linear 2×2 system:
x cos θ1 + y sin θ1 = r1 x cos θ2 + y sin θ2 = r2
that is AX = b, where
A = [cos θ1 sin θ1] b = |r1| X = |x| [cos θ2 sin θ2] |r2| |y|