Yeah, great question. What's is your intended use?
I know with my M series Studebaker 2 speed axle the ratios are 6.66:1 and 5.66:1 with no other gearing options. Neither choice is very good for highway use. I guess if you were going to use an overdrive unit or trans that would make the 2 speed rear more attractive if you needed it for highway use.
I'm not that familiar with the cummins specs. I would think that you would want to research the best rpm ranges at which the cummins performs and compare that to the gear ratios of your 2 speed axle along with your tire size to determine where the cummins would operate with your setup. I do know over revving any diesel is bad news.