If you think about it for a minute - of course Boeing designed it to land on just one A/P - it's called 'fail-safe' or 'fail positive'. It allows for the case where one A/P fails just before the flare - what do you think would happen if the flare mode failed in toto, just because one A/P fails?
You will get no Flare Armed indications with just one channel selected, or if one fails, but it will still flare, just fine!