Installing more than one battery pack in Macbook?

From a battery controller logic standpoint, I am fairly confident that is not possible, in the sense that the result will not be a stable configuration. The reason is simply that there is no mechanism to handle such a scenario on a logic level.

My understanding is that a MacBook battery has some simple logic and communicates the charge level and other battery internal states to the charging controller on the mac logic board. If you parallel-wire two batteries together, the logic on those two batteries will generate cross-talk when the logic board controller tries to talk to the battery.

The logic board sends an enquiry, which will go to both batteries, and the two batteries will answer back at the same time, possibly with different answers. That will not go down well in the logic board controller.

So I think the only way this would be possible is if you could connect the two batteries to a hypothetical 3rd party battery controller board, which in turn would be connected to the MacBook logic board, mediating the communication between the logic board and the batteries.