Thanks for the reply BitMore. I've come across a whole bunch of schools, mostly US based, advertising for these 24 hours TR courses although they don't at first tell you the number of hours until you dig a bit deeper. They all seem to split 12 hrs left and 12 hrs right seat. I've been a CFI for a number of years and have rarely come across a student able to upgrade to a light piston twin in less than 12 hours, let alone take on a 144,000 LB monster in the same time. Your experience seems to support the notion that a 12/24 hour TR course sounds questionable.
I wonder if anybody here on Pprune has done one of these courses?