The German Battery Top light 4D Senso Taillights I've Owned have a 3 position switch . Sensing darkness and motion the light comes on automatically..

Day time , in that mode it comes on in a Tunnel then back off when back out ..

since flashing mode is the lowest battery draw is it a lot of bother to switch on your removable Battery lights before you start?

Any switch thing you dream up has to be unplugged to pocket the lights to not get stripped off the parked Bike.

maybe a common battery Pack mounted elsewhere on the frame ?

I made up a 5D cell rechargeable battery pack 20 years ago and ran a cheap Halogen bulb headlight and a bulb taillight, on the Mudguard.

Now If you instead Install a hub Dynamo and wire the taillight, then there is a switch to turn the Headlight and the taillight that is wired into It, off.