The 314 group of regulations are to do with division of installation. What I am suggesting is that the armour of a multicore SWA covering live conductors of more than one circuit is perfectly acceptable. Think about earthed metallic conduit where the conduit forms the protective conductor. It has always been acceptable to have more than one circuit with the conduit. The multicore SWA is akin to that. Overall, 2 cables will of course give you more armour, but more expense, to my mind unnecessarily.
Thank you for the point about equivalent conductance for protective bonding conductors. I had forgotten you cannot just apply the adiabatic rules as you can for protective conductors!
Regards Bruce