A feature of Mallet-style articulation (both compound Mallets and the latest simple articulates) is that the forward engine frame is articulated with the rear engine frame, so the pull of the forward engine is transmitted at roughly axle level, and without stressing the boiler or associated pipework. That's missing here. Major, MAJOR, change in the structural/mechanical design! Time-travel back to 1913 and show this to the designers at Alco or Baldwin and they will think you are an escaped lunatic.
Which said, there are lots of different schemes for building articulated steam locomotives, and some of them did involve powered "trucks" that were (like the bogies on a typical diesel) connected only via the superstructure. ( Bulleid's "Leader" (for Southern Railway / British Rail) and "turf burner" (for CIE (Irish Rail)) come to mind, though not happily. )
(And lengthening boiler tubes isn't always a good thing: increases the heating surface (good) but also -- because of the increases friction involved in going through longer narrow pipes -- hinders the passage of hot gas through the boiler (bad, very bad). Most designers of (coal burning: oil burners seem to have had different dynamics) in the last generation of American steam tried to keep the boiler tubes relatively short (say, 19 feet), filling the gap between the rear end of the tubes and the firebox in a long-barreled boiler with a combustion chamber.)
Do follow up Desertdweller's suggestion of a Garratt. They look very strange to American eyes (Alco had an American license to the design, but didn't find any customers), but were built in many sizes and configurations, both narrow gauge and standard gauge and were used successfully by railways on five continents. One built to the North American loading gauge (= clearance envelope) could have tall domes and a prominent stack, and there are enough options in water-tank and coal-bunker design that you might be able to get something that would appeal.
(I hope you're not offended by my first paragraph: I enjoy speculative locomotive design, so please don't hesitate to suggest more. I'll feel free to criticize them viciously if I don't like them… but with a smile and what I hope you'll recognize as a friendly attitude.)
