There are several interesting points in this thread..
(1) Do doubles require special training? Depends - if you double up Aluminum 80's and dive in open water, probably not. On the other hand, if you double up Steel 104's and want to dive in a "technical" environment, then there are gas management and buoyancy considerations related to doubles that can be life-threatening without training on the use and the effect of the doubles.
Diving big steel doubles even in the most benign environment introduces buoyancy considerations that must be carefully considered.
(2) Tank sizes - Litres vs Cubic Feet - sort of an apples and oranges comparison. The Litre rating system is a measurement of the interior volume of the tank, while the cubic foot rating is expressing how much gas (measured at 1 ATM) can be shoved into the tank at it's working pressure (approximately). So, for example, if a LP 11L tank is a "72" at 2475 psi, a "standard" 11L tank rated for 3000 psi would now be an "85" and a HP 11L tank rated at 3500 psi would be a "100."
In other words, the "conversion formula" must include the working pressure to do its job.
(3) As far as I know, there's no "doubles" course - but training in their use is included in almost any technical course.
Rick