how to use velocity in two axes at the same time (X and Y axis) ?

You can use body movers.

local debris = game:GetService("Debris")

box.Touched:Connect(function(partTouched)
    if partTouched == ball then
        local bodyVelocity = Instance.new("BodyVelocity",ball) --create a body velocity and parent it to the ball
        bodyVelocity.MaxForce = Vector3.new(math.huge,math.huge,math.huge) -- Determines the limit on how much force that may be applied to each axis
        bodyVelocity.P = 1000 -- Determines how aggressive of a force is applied in reaching the goal velocity
        bodyVelocity.Velocity = ball.CFrame.lookVector * 100 + Vector3.new(0,50,0) -- make the goal of the velocity 100 studs out from where the ball is looking and up 50 studs.
        debris:AddItem(bodyVelocity, 0.5) -- deletes the bodyVelocity after 0.5 seconds. You can adjust this value and the other values above to your liking
    end
end

If you have any issues let me know.