There were a few Australian Horse Conchs under and around the jetty,on sand or near low rocks. I'm sure I've not re-photographed the similar but larger shell (which carried 2 visible chitons).This is another shell (probably also an Australian Horse Conch). So I wonder if this is a common feeding behaviour for the local horse conch population?
Do they raid chiton 'hotspots' e.g. the rocks under the jetty cut-water whenever conditions suit (this was on an outgoing tide,calm sunny day with good viz)?
I'd be surprised if the chitons were hitching a ride deliberately,so I'll assume the ones in that other submission's photo that looked to be tenuously but almost safely out of harm's way (just clear of the back edge of the conch's foot and thus feeding mechanism) were merely postponing the inevitable.

Craspedochiton umgaziana (Koch 1951)
Koch, H.J. (1951) A new chiton from South Africa. Proceedings
of the Malacological Society of London 28: 211–212, pls 23–25.
Schwabe. E. & Els.R. 2019. The genus Craspedochiton (Mollusca: Polyplacophora: Acanthochitonidae) in South African waters: what do we really know about it? Archiv für Molluskenkunde Vol 148p 73-92.