Well, here is the deal, it must be wired in PARALLEL, rather than series. If you wire your batteries in SERIES, ONLY the VOLTAGE would double. However, if you wire them in PARALLEL, ONLY your mah rating will double
I know this because Marc Kim(Leegeddy) wired me a special custom 3 x Tamiya RC Connectors>3-Amp Fuse>4-Pin XLR for my 722 dummy cell powering method. He wired them in PARALLEL, so that ONLY the mah rating would triple, and not the voltage. Therefor, if you wired them in SERIES, I have would instead have a 21.6volt battery, with a 3000mah rating, which would be very bad for my 722
I instead have a 7.2v/9000mah battery instead
So in order for (2) 9volt batteries to be wired like that, he sired them in PARALLEL, otehrwise, you would ahve an 18volt battery
like i said, wire din parallel, ONLY your mah rating goes up. I would say that is what he did.
so you have a VR Box, that is wired in PARALLEL to allow (2) 9v dvd batteries to be used