@101100 suggested a ‘Reflector’ part, to send balls from either side back to the same side that they came from. @JohnZ Designed and 3D printed a one. I don’t have a 3D printer handy so improvised using only parts in the box.
My new reflector uses two of the high-friction washers to attach a ball to the bottom of a gear bit. The extra weight keeps it centred.
My previous reflector used a crossover, cog and interceptor, and has special powers: My second invention of the week - cog controlled reflector/crossover/ball trap. However, it’s also nice to have one that doesn’t use up three such useful parts.