It's not clear to me what you are asking. What does the intersection of two surfaces have to do with the speed of a point? Is the point moving along the intersection? In that case, its velocity vector will be either pointing in the same direction or pointing opposite to the tangent vector of that curve. Use "pointing in the decreasing x values" to decide which, divide the tangent vector by the its own length (magnitude), and multiply by the speed to find the velocity vector.