I don't know how the circuitry works. If it senses voltage and acts accordingly hooking the two bats together would work but on the other hand if the circuits sense current flow it could be confused since with two bats it would be seeing double the current.
Unless someone more knowledgeable comes along to assist I think you are on your own to try it or not - depends on the cost of frying something.
I would think there is a fuse of some sort to protect from overloads but whether there is or not or if you could ever find it if it blew is another unknown.
Off the top I'd guess it would not work but I've been wrong many times before :-)