Roller-coasters, like all tapped/adjustable coils, can sometimes suffer an effect called 'suck-out', where the unused part of the coil happens to have a self-resonance at the working frequency: if the coil is a transmitter 'tank' you can get some seriously-high circulating currents as a result, causing loss of output-power and much local heating.
One way round this is to short-circuit the un-needed part of the coil, but again this can mess-up the 'Q' and reduce the inductance.
Short-circuiting _part_ of the unused winding - the part furthest from the part-in-use - helps reduce these unwanted effects. One wheel for the tap, the other as a moving-short?
See
http://g3ynh.info/comps/Vari_L.html and note mention of the MFJ "Self-resonance killer"
that shorts different bits of the coil under different circunstances.