The past use, IMO, was confusing about double vs doublED. Dd...when in fact both are in reality, "doubled", just that one moves while ... doubled...the other does not.
Also an important distinction and one being overlooked, is the two fixed ends of DRT vs a fixed end / free end, of SRT.
Give this some thought. 2 or tandem SRT has 2 fixed ends and 2 free ends probably very far apart, DRT has 2 fixed ends at the climber. Where the ends are or are not connected, is huge.