I know that Merc requested all the BH 2 Speeds back and was providing the cost to cover the re-rig to a Bravo in place of the BH back then. I know "Powerboat Transmissions" aka Richard Peat in Pasadena has the B&M 2 Speeds now but they are 16.375" at their shortest.
The clutch needs to be operated via, air cylinder, linear actuator, or some hydraulic slave system but that takes coordination on the operator. A transmission controller or an internally valved transmission that could do this when shifted would be ideal but .... The BMW has this system with their SMG
Counter rotating is not an issue as we do that in the drives, saves engine room space and cg..