insurances are odd things as most of mine have had a reason to decline but always paid out when i'v needed them. HOWEVER ifyou knowinglyput down 1 person only and don't include pillions but you take them, they have every right to void the policy and it might addect the other person claiming too if they're left unable to work.
in cases like this i take upmost responsibility to ensure the pillions covered as they are my responsibility no one elses. i'd hate it if they got screwed over because of me and they didn't have any idea why.