I surely would like to read these reports. Can you suggest a source?
They are available from a number of sources, I think including hathitrust. Copies, and at least one analysis of the results, are in the T1 Trust file repository. I think Joe Burgard posts here, and he can recapitulate some of the points he thought most important.
Further complexity of the run, for me, is sort of up in the air. I can see that it is possible that it would be more complex. But I am not sure it necessarily is.
Here is a picture from Bill's Pennsy Photos that shows some of the problem:
Note all the 'stuff' that the rear outside supply pipes have to navigate to get to position, and the fabricated bends to get piping with good flow smoothness that can be physically removed and installed for maintenance.
The B&O locomotive did all this differently:
Here the valve cylinders are kept inboard, like early locomotives with piston valves and eccentric-driven valve-gear. I do not have (but would like to see) a drawing that shows the pipe routing, and a description of how the rear valves were inspected and serviced when necessary.
An issue mentioned as significant with the forward-facing piston rods was that they were comparatively easy to 'nick' with bouncing ballast or coat with grit. This is relatively easily fixed by giving them vented 'boots' like those on shock absorbers of 'mud trucks' with extreme lift kits. There is also some accelerated wear to crosshead surfaces that would need to be addressed with good modern materials.
I figure my rigid wheelbase would be about the same. I am increasing driver diameter by 3", but I am eliminating the 18" between the first and second axle on the UP.
The problem isn't so much matching the wheelbase on the Nines as it is dealing with the pronounced overhang and polar moment of inertia with a design that has a larger and much heavier firebox and chamber structure. Ideally there is a geometric formula for positioning the two axles and pivot point of a Bissel trailing truck (note sp.) so that it doesn't exert unbalanced lateral flange force when following curves relative to the rigid driver wheelbase. In practice the axle locations may not be, for weight-distribution reasons, where this formula puts them (and Timken for example designed for this by providing lateral motion for the first trailing-truck axle via hardened steel rollers and wear plates). I would be concerned about how you effectively control the long lever arm and hence fairly extreme overhang at the rear of the locomotive chassis in particular.
I want to stay away from articulation, because then the individual engines would eliminate the possibility of reciprocating balance.
In a sense, the engines of any Mallet type are 'duplex' in being physically separate. What I think you mean is that the hinged engine can't be given effectively low overbalance because of relatively uncontrolled yaw -- that was certainly true of older articulateds that hinged in both horizontal and vertical planes and did not have compliance on the swing of the forward engine. It is less true of the post-'30s designs, and of course the tendency with any long, heavy modern North American engine is to restrain yaw and its couples with inertia; the Mallet chassis itself being relatively insensitive to induced yaw from low or zero overbalance in the rear engine, the engineering comes to apply to the hinged engine.
Here the fun starts if you want to perform Deem conjugation - of course this can be done right across the hinged joint, if necessary by using a double pin and yoke arrangement similar to a Cardan-shaft universal end or clevis. Then you use the methods Glaze used to stabilize the forward end of the J class, both with stiffer and more positive lateral on the pivot of the lead truck and with spring and dashpot arrangements to damp fast accelerations in front-engine swing without resisting necessary swing on curves. It is not difficult even with '40s technologies to make this adaptive to a significant degree, or actually make it servo-assisted.
I figure, at a minimum, there would have to be twin exhaust stacks, split front/rear [...] When I said twin stacks, I was thinking about the classic setup on the UP articulateds. Both stacks would be in the "standard" location.
Note that many of the 800s, including 844, have two-stack arrangements for better draft (note the arrangements for the nozzles relative to the exhaust passages in the cylinder saddle). There is no direct requirement for this; N&W for instance successfully provided a kind of 'rosepetal' nozzle in which the openings for exhaust steam from one engine alternated with the other's. Then you would just use the appropriate larger stack diameter and proportions needed for actual draft-producing mass flow.
Of course I'm not going to tell you NOT to use a twin-stack arrangement, just to proportion things so they work at least cost and most optimal back pressure. I don't really know of any 'side-by-side' arrangements that actually worked well (one British locomotive used triple thin stacks, but that was for making the engine harder to follow during air raids) but you might be able to keep the smokebox length controlled by using parallel Giesl ejectors and some internal baffling to keep the flow patterns good.
Your last paragraph does, indirectly, bring up the possibility of cab-forward operation. It's a thought.
I actually can't take any credit for that. ATSF of course is famous for its 6-4-4-4 oil-burning duplex design, which was to have been cab-forward, and the German 05 003 in particular was built cab-forward with coal firing (admittedly pulverized). A concern I have is that operating a twelve-coupled engine stern-first puts an awful amount of mass on an awfully long lever arm out ahead of the lead and second driver pairs, where it might not be controlled very well by a pin-guided truck of the usual cab-forward style.