I would not consider the split torque design to present an "order of magnitude" greater difficulty than that of the CH-53E. However, simply growing the existing CH-53E configuration to accomodate a 30% increase in power presented some serious design challenges, even without weight concerns. The 53E MRGB uses a spiral bevel 1st stage and planetary 2nd/3rd stages. Early in the 53K program NAVAIR asked Sikorsky to perform an MRGB trade study comparing an improved planetary design to a split torque design. NAVAIR wanted to determine if the advantages of the split torque configuration were enough to justify the greater development risk. Among the advantages provided by the split torque configuration were lower weight, more space for increasing the main rotor shaft diameter, and a more efficient main housing structure.
If you are interested in reading more about the 53K MRGB design trade study work done by Sikorsky, there is an excellent
AHS technical paper from 2008.