I mean, like you can buy SIM-adapters in which you can insert two normal (or modified) SIM-cards, there could be "three pin to two pin" battery extender which you could put between the original 3 pin connector and the original 1st battery, and then there would be second 2 pin connector (with some kind of rubber band) where you can connect the 2nd battery on top of the 1st. (I should draw maybe).

The problem I see is there is no much space left when the 1st battery is inserted to the battery hole, so the first battery should maybe be modified anyway.

The idea would be not need to modify the phone itself or batteries themselves at all, but still get two batteries parallel connected and used.