How I did it was put the two halves as prop_phsyics in position (stuck together) exactly where I wanted them to be cut at. I made a
point_template with both halves listed. This makes them not exist on the map in game until the template is called upon.
When the cart is cut, I kill off the original cart prop and force the spawn of the template which will spawn in the two halves. I put a tiny little invisible wedge in at bottom of the split so they'd do a nice satisfying plop apart from each other, too.
Hope that makes sense!