don't think the PT6 is a twin spool. it is a single spool with a free turbine.
the free turbine is what drives the gearbox and prop which are not mechanically connected to the spool, hence the term free turbine.
because the gearbox and prop are not connected to the spool, when you turn over a PT6 to start it, the starter generator only has to turn the spool which is also known as the gas generator, and not the spool, gearbox and prop so the prop can remain feathered without putting any strain on the starter generator.
StraightLevel