Even if the theory say this is possible, it is not practical in real life. To manage this your batteries needs to be exact equal in voltage. If not the battery with the lowest voltage will 'short' the one with the higher voltage and draw power from that until the capacity/voltage is down to the low level. This will in the best case waste a lot of the capacity, in the worst case the batteries may run away into heat and maybe overheat. It depends on the battery techology.
In both cases the you will not get the double capacity you wanted.
One way to do this is to isolate each string of batteries with a low drop diode, connected in a common point that is the supply voltage. This way each battery(string) will supply it's power in parallel with the other, without feedback to the other batteries.
The voltage will be a bit lower, depending on the diodes in use, but you'll get max power out of the batteries.
TOK