Let me make a few suggestions (I've been reviewing South Dakota history since I know it less well than I know ND history):
1) Early in South Dakota's statehood (1889) there was a large push to have the Sioux cede their reservation lands to the state as it was feared by the President Harrison that the state would not actually be viable with so much of its territory controlled by the tribes. The end result was that the Sioux gave away a bit over half of their promised land.
With a larger state of Dakota this is going to be much less of an issue and, although I suspect some lands will be negotiated away from the Sioux, but nowhere near to that same extent.
As a result, the southern parts of this state of Dakota are going to be less settled by whites than OTL South Dakota.
2) The territorial government had already moved from Yankton to Bismarck before the states were split in OTL. In this ATL it seems likely that Bismarck will remain the capital of Dakota. I am guessing that this means that the state government is going to favor development in the northern parts of the state.
In OTL, the Great Northern and the Northern Pacific Rail lines were established in North Dakota prior to statehood. Meanwhile the Chicago and Northwest and the Milwaukee Road were the two dominant lines in the South. I believe in a timeline with a united state, that Fargo and Sioux Falls jockey for the position of the dominant urban center in Dakota with Fargo eventually winning out (however, that may just be my own personal bias, as I lived downtown for eight years

)
My logic is as follows: With Bismarck being the capital of the entire state, it is going to become the dominant urban center in the west with no *South Dakota community able to compete. Eventually, Bismarck will become a rail hub for those western farmers needing to get their crops to market. Since it is easier to run rails from Bismarck to Fargo and, from there, to the Twin Cities, Fargo will likely eclipse Sioux Falls as the dominant rail hub.
As a result, Fargo (and Bismarck) will likely be more populous than they were in OTL and, due to the larger reservations, *South Dakota is slightly less populated.
3) The State of Dakota is going to initially three land-grant universities from the get-go since the University of South Dakota at Vermillion and the University of North Dakota in Grand Forks were both established by the territorial legislature prior to statehood. Meanwhile, North Dakota Agricultural School (NDAC) is likely still founded in Fargo on schedule due to the fact that plans had been in place to create an Agricultural school in the northern parts of Dakota since the territorial days.
Just some development thoughts off the top of my head, please feel free to disagree or comment. An interesting side note; the fact that development and population are going to favor the North is going to leave many in the south and the west feeling overlooked. They could actually be more welcoming to the message of the NPL (or similar organization. Possibly, in the ATL, A.C. Townley stays with the Socialists) than SoDaks were in OTL. This could become more entrenched with the coming of the Dust Bowl and Great Depression.