Thanks for your feedback, chaps.
After posting that last night, and after some Googling and the girls talking to friends, 'we' managed to get the free Fortnight up and running with a split screen.
It's not as bad as I feared, but still frustrating that for the vast majority of multi-player games, it's a case of one player per console.
When I say 'frustrating', I mean 'shit, I wish I'd have done proper research first'