Yep, always use two lights and have recently swapped to rechargable batteries, so I can just regularly top them up, rather than wait until 'normal' batteries go flat.
I got back from a ride recently and while both lights were on, they were significantly dimmer than they should have been. They were bloomin' bright when i started the ride though, which made me realise with LEDs it is quite hard to judge when the batteries are dying.
Decided rechargables are the way forward.
And yep I always use two rear lights; one flashing and one solid. Be very unfortunate if they both ran out completely on a ride.