Mate-these batteries you have are 12v -
coupling them together makes 24v-this will blow your bulb unless you upgrade it.charge them both up and take them both to be used seperately.
i use 2 12v 7 amp batteries mate wired together to mate 12v 14 amp,i do this the same way as deben do it,wire them red to red and black to black withe wire coming off 1 battery to connect to lamp,it does'nt blow the bulb or the charger mate.this is my set up mate i done it myself.
cheers lads, thanks for the help! saves me going back to the van and changing batts