I would like to mount two batteries on my bike and have them both simultaneously connected, just like having a battery that's twice as big. Using the current style batteries, I would mount the second on the water bottle bosses on the underside of my downtube.

How about a battery package that could mount on the front of the bike, like touring luggage. It could be either in two sections like lowrider panniers, on a rack like the Tubus Duo or Tara, or in one section to mount above the wheel on a constructeur or randonneur style rack. There would be some challenge involved in routing the power cable, but that's not insurmountable.