Individually each cycle may be able to reach the efficiencies you state, but put into combination (by that I assume you mean employing the steam portion to use the waste heat of the Diesel engine in a bottoming cycle), you can never reach any significant improvement over the Diesel engine alone. The reason is the temperature of the waste exhaust heat is relatively low, and therefore so is its "exergy" or availability to do useful work.
Secondly, when tossing around efficiency figures, we're always dealing with peak values. Even a Diesel engine with a peak efficiency of 40% is not the problem. The problem is that, like every other thermodynamic cycle, the engine is rarely operating at a point of peak efficiency. Far from it. Whether Diesel or otherwise, the power at the wheels is less than about 20% of the energy latent in the fuel.
Even doubling the peak efficiency of any engine process will at part loads bring relatively small improvements in the whole scheme of things. On top of that, you add all the extra complexity, bulk and the dreaded C-word -- cost -- with a small steam engine, and you can understand why nobody is beating down doors to get this to consumers' hands.
BMW has demonstrated a small auxiliary steam engine running off the exhaust heat, but it's just that: it's a technology demonstrator, not something that will be cost effective anytime soon. And its coupled to a gasoline engine. With a Diesel, there would not be enough exhaust heat to justify the steam bottoming cycle, and Diesel already has a cost disadvantage to begin with.
Nice idea for laboratories and maybe massive combined-cycle stationary generators, not quite ready for prime time automotive production.
