DH had brilliant idea. No, really. We've snipped off the first 6 inches of the Cube run, turned it round, and then permanently clipped it to the fixed run. We can then connect the Cube run to this "docking station" by means ofrun clips. Or cable ties. Or something.
The ground is terribly uneven and slopes away by many inches, so we're having to put narrow slabs all round the Cube run, and build a little runway of bricks for the wheels. And of course we're (=DH) having to level it first. I'll post a picture when we've levelled it out.