You only need one. I can't imagine that is 20 kgs.
I would advise you to check the front mounting bolt on your anti tramp bar for bending and causing the front mounting loop on your spring to develop hairline cracks. On my Roadster the spring broke and the mounting bolt snapped and it was all caused by only having one anti tramp bar putting side pressure on the mountings. New better quality springs and removing the anti tramp bar ( not needed unless racing and two would be better) solved the problem. Don't have any tramping even at full gas.
Jim.
