No, not if they only use simple steam....full pressure steam to both engines. A Mallet, strictly, even though it is articulated, uses the same pulse of steam twice, first in the high pressure (smaller) cylinders, and then in the larger expansion chambers, at which it is emptied into the smoke box and goes up the pipe, just like twin cylinder and three cylinder locomotives.
IOW, a 'Mallet' is only ever a 'compound' system, using steam twice. Articulated engines, like the 2-6-6-2 run by the CSAR, one of a very few examples of non-compound 2-6-6-2 engines, were simple expansion.
In case anyone is interested, many early steamboat engines were triple-expansion; steam used in three different expansion chambers, with the third one being rather large...boat size.
Js I will look again at my Riv version to see if 2 diff size cylinders..But, as I say, I believe it's correct to call them all articulateds..Or for that matter all 'mallets' / 'mallies' based on the inventor's name. Eventually some were made compound and some were not along with the ensuing various wheel arrangements and builders of...from Old Maude to Big Boy.
But for me it's always been the y6b ! 🛤🌄🛤 M
BTW. would love one on those 2-6-6-2Ts Dennis has, above DCC/sound... always took a liking to those babies as well....
I looked at my examples I have of the y6b and they’re both mallet compound set ups. I did a little research on them and they would start out as simple expansion then they would turn a valve and make it run compound after about 10 mph. So I’m guessing simple expansion is not as “efficient” as compound but produces more power
There is some history behind the term "Mallet" and articulated prototype locomotives were often called "mallets" whether they were, as mesenteria pointed out, compounds (true Mallets) or not. You can look up "Mallet" in the attached "Dictionary" file and read more about how the name was often misused.