OK. Though I don't think they, or much else, was converted to dual brake, it was a challenge to fit both sets of gear, plus the complex rigging to the blocks, into the underframe. St Rollox certainly did one, it was a Mk1 dual heat BCK, which when dual braked gave one of everything, class, van, brake and heat, and was stationed at Carstairs from 1974 just in case one of the portioned workings turned up missing a key vehicle for the continuation. Sometimes it was inside the old loco shed, sometimes outside in front of it. Must have been an interesting project for both drawing office and fitters.
I wonder if it was either part of the 1974 plan, or a rush job after some early days events, and how often it was needed.