I pose this question to Devil 49 who quotes the "book" to us about intentional autorotations in twins being an improper technique. Devil...how do you propose to conduct training to prepare pilots for those interesting situations that occasionally crop up in "twin" helicopters such as a main drive shaft failure on Bell 212/412 aircraft for instance? Do you reckon this will happen with the collective full down and at the correct autorotational airspeed, into wind, with a beautiful landing area under you with plenty of altitude to prepare the aircraft for landing?
I endorse your idea of computing the data needed to confirm that the Nr is correct during the practice autorotation but from there I fear you and I take different paths. As Twin-engine helicopter pilots we quickly become complacent with the reliability of the aircraft and its ability to remain airborne following an engine failure after we are in flight. We must remember that other failures may render us incapable of maintaining power to the rotor system such as the drive shaft failure, dual engine failure due to fuel contamination. The better we train for these odd events will greatly enhance our ability to cope with the real thing should it happen. I don't know about you, but I am the happy recipient of many hundreds of practice autorotations in multi-engine helicopters and more than a few to touchdowns from BO/BK's right the way up to the CH-47 Chinook. Remember the saying, "You do in real life what you do in training." I therefore submit that training should be done as close to real life as is safely possible. The operative word is "safely". I have read the same manuals and understand why the advice is given there. Perhaps, we should request Nick Lappos to weigh in on this and explain why manufacturers put such cautions into the flight manuals....usually it is for legal reasons and not for any mechanical or technical reasons. I also think Nick may have done his share of autorotations in multi-engine helicopters over the years....but of course having been a former Cobra pilot he probably was well experienced in doing touchdowns.