Single pilot/multi crew
Hi
You need to hold a JAR CPL to be able to command an aircraft like the Twotter, having said that most companies now require you to have at least a frozen ATPL. At least that is now the case with my company. Don't know the requirements of other ICAO states I'm afraid. The reason for the frozen ATPL is so that whoever employes you has the flexibilty to transfer you within different fleets.
Potkettleblack is wrong in stating that you gain nothing, you can gain valuable experience plus if you can get 500hrs in an aircraft like the Twotter flying multi crew and get your employer (in the UK) to validate it as such it negates the the requirement to do an MCC course, at least that was my experience.
Hope this helps.