For scale Lionel or MTH steamers, the drivers are locked in whatever position they're assembled in as there is a inflexible gear train and one motor powering both sets of drivers.
Talking about prototypes, Mallet locomotives have a "simpleing valve" that starts them out in simple mode. I know the Legacy Y6b starts out with the articulated sounds and you can hear it shift into 4 chuffs at a certain speed just like the prototype would do. I would think most Mallet locomotives would have to similarly start out in simple mode just to get moving as that's where maximum tractive effort would usually be called for. That being the case, even the Mallet would have the articulated sounds at startup.
From the Catskill Archive Website, the simpleing value appears to indeed be a standard feature for the Mallet style locomotives.
In order to enable the locomotive to develop full power at starting, it is necessary to provide means for admitting steam direct from the boiler to the low-pressure cylinders. In the Baldwin engine, a small pipe is run from a starting valve in the cab, to the receiver pipe connecting the high and low-pressure cylinders. By opening the starting valve, steam will pass direct from the boiler to the receiver pipe and thence to the low-pressure cylinders, and the locomotive will develop a tractive force up to the limit of its adhesion. This device is recommended because of its simplicity and reliability in service.
