Hi,
My first attempt at kit-bashing (imagineering) taking two (say 494 and 495) and making a Garratt:
I think adding a course or two of a second boiler onto the first would be better for steaming.
I do not show a fishbelly bridge/flat car spanning the two engines yet and I am using two standard K37 cisterns - the left one (next to the cab would be for coal and water and the one on the right would be for water. I actually think that use would be short distance Chama to Cumbres so the larger tank and tender would not be needed.
The first six (6) of the K37's were about 1928. If the second four were instead two Garratt types, and used over Marshall Pass with 4% on both sides, larger cisterns would be doable. An advantage of the higher cab and lower tanks would be visibility.
I have not shown a redesign on the fire box - it would drop lower.
I joked about a sort of cab center Mountaineer type using two full K37 boilers but It would make the item too long for curves I think.
I do not know much about D&RGW SG steam but something with a larger boiler might have been available for the Garratt.
I started with my file "197910 mr p72-3.jpg" which is Model Railroader October 1979 pages 72-3.
FWIW.
Doug vV