Thought I should update this just in case anyone's been looking (probably not so much in summer!).
I got a good deal on a B&M Cyo plus. This has a switch and standlight. The rear light connection is switched AC. I bought the cheapest standlight equipped rear light I could find with no switch.
The front light comes on and off with the switch as you'd expect. The capacitor holds charge for weeks on end so you pretty much always have light when you need it.
When you switch off or stop the rear light will stay on until the capacitor drains. This is mildly irritating but slightly less annoying than having to faff with two switches. The other downside is you have to cycle for a while before it'll stay lit when you come to a stop (my first turn on my commute home is only a couple of hundred metres from the office so there's never enough charge to keep the light on).
As for drag, I can feel the change when I flick the switch while cycling. Not significant but enough to make me glad I can switch it off!
Overall I'm extremely pleased with my dynamo powered lights and wouldn't go back. Still can't understand why no one is selling lights as matched pairs with a single shared switch and capacitor though!
Thanks for all the answers.