If I understand your description correctly, your problem is that the battery pack is producing 3 volts, not 6, since you wired them in parallel, not series.
Maybe a description would help: Batteries in a normal flashlight ( end to end, in a line ) are in series, the voltage of each battery adding to the previous. So if you have 2 batteries , each 1.5 volts, in series you get 3 volts, they add up.
Another way to connect them is in parallel, where the positives are all connected together and the negatives are connected together. Here, 2 batteries each of 1.5 volts, would still produce 1.5 volts
- so -
Each battery pack has 2 "c" cells, at 1.5 volts each, wired in series, producing 3 volts. You connected 2 of these packs in parallel, resulting in 3 volts. The parallel wiring will give you a longer running life, but not increase the voltage. You need to connect the 2 battery packs in series to get 6 volts. Take one balck wire on one pack, connect it to the negative of the adapter. Take that packs red wire, connect it to the black wire of the other pack. Take the remaining red wire, and connect it to the positive of the adapter.
Hope this helps. If I misunderstood you ignore the above, lol 8)