The requirement is for the 2 NDB's (navaids) and DME or GPS.
To conduct the approach you require the equipment to utilise the navaid systems. One ADF and one DME or GPS receiver will do that.
Also ERSA indicates Bolinda has a night range of 30 nm and Rockdale a night range of 20 nm.
A navaid approach designer might have an opinion on whether the NDB range is a factor in the design of the approach and the requirement to use both NDB's.