Actually 2 turbine/shaft is fairly normal. The only reason to have 1/shaft is to allow twin prop redundency in smaller ships.
As for power the LM2500 can now put out 30,000 HP, with growth potential to 40K. Two of them coupled would supply as much power as you can put down a single shaft. An Iowa is going to need 4 shafts whatever the power source.
Well, No.
As originally built, the BB61 class had 212,000 shaft horsepower. Four LM2500s would provide 120,000 shp.
Its common to have two engines per shaft, but it isn't common to rely on running both of them on the same shaft together. I've been on many gas turbine ships, and its been rare to have both up and running at the same time.
That said, I prefaced my statement by saying you would need two coupled per shaft to get the shp required. So for proper maintenance, you would require four per shaft.
With modern electric motors, it might be better to pull the shafts altogether, and go to pods. Gas turbines could then power generators.