If you take the simplest design then the second object would be launching into the ground, which is basically what they're doing with the counterweight. Alterna

If you take the simplest design then the second object would be launching into the ground, which is basically what they're doing with the counterweight. Alternatively you could have a contrarotating system, but I'd then be worried about two high speed objects travelling close together. The best solution would probably be a metal counterweight, so that you can regeneratively decelerate it from it's exit port.