Sikorsky xmsn patent
Thar Sikorsky patent describes a two speed transmission. It gets variable speed the same way the V22 does, by changing engine speed. Turboshaft engines tend to have a narrow range of usable operating speeds. Usually something like 70 to 100 percent under ideal conditions. A two speed transmission would essentially double that range.
However, that transmission schematic shows two types of clutches, a friction clutch and a sprag clutch. And each of those clutches will likely have some reliability issues the way they are being employed.
The friction clutch is being used to synchronize two shafts when the high speed gear ratio is to be engaged. The friction clutch must provide slip until the two shaft speeds are equal, which requires the clutch pack to absorb a great deal of heat energy without failure. That requires a large mass of friction material like CRC. Also, being a device that transmits power solely through friction and not having a reliable way to monitor its condition, good design practice would probably dictate that the friction clutch should be mechanically shunted (with a dog clutch or some similar positive engagement device) once it has fully synchronized.
As others have noted, driveline shaft dynamics can be a real nightmare. The varying and aggressive friction characteristics of carbon materials makes it near impossible to smoothly modulate the slip of a carbon clutch pack. This issue was a real problem for Rolls/Allison on the F-35 lift fan clutch.
As for the sprag clutch, they do not like to operate under overrun for extended periods, such as this Sikorsky design concept would require.
Since they have two engines with an interconnect, they would be better off doing a fully synchronized shift using dog clutches with the speeds controlled by the FADEC's.