Double (twin tail or Y) lanyards allow a climber to stay connected, and should have double action connectors. They are easily misused: no back hooking, not wrapped around the body or passed between the legs, chest connection never above the highest attachment point, not suited to frequent use, and ladder or structure attachment points must withstand the arrest forces. Users should be trained. Side loading of connectors sliding to the low point of a rail can be controlled with soft slings or hooks rated for side loading.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.