Microsoft confirms that the Surface Pro 5 does not support USB-C charging. It charges only through the Surface Connect port.
Ref:
Surface charging requirements and power supplies for Surface Pro
Best alternative: a 44W Surface Connect replacement charger with a built-in USB-A port. This is the closest thing to a "second original charger" rather than a workaround — it uses the same magnetic connector, same voltage/current (15V, 2.58A), and won't have any of the compatibility uncertainty that comes with USB-C or car chargers on this model.