Why might some turbine engines have more than one turbine wheel on a single shaft?

Having multiple turbine wheels on a single shaft allows for more efficient extraction of energy from the exhaust gases produced by the engine. Each turbine wheel can be designed to operate optimally at different pressures or temperatures, enabling the engine to maximize energy recovery from the hot exhaust flow. This design can enhance the overall performance by increasing the amount of usable work derived from the exhaust gases, thus enabling higher power outputs with the same engine size or improving the efficiency of the engine operation.

In a turbine engine, the ability to harness more energy from the exhaust stream directly correlates to improved thrust or power generation, making it a critical component in achieving desired performance characteristics.