Hi Simon, I have 2 210L water butts that each had a tap on the bottom. I 1st of all connected the two taps with a short piece of 1/2" hose, but i found that it was very slow at transferring between the two and i ended up having to move the submersible pump from one to the other water butt.
So what i did was to remove the original tap assembly from the water butt's, it was just a screw fitting with a plastic nut on the inside. the thread on the taps were exactly the same size as a 22mm water tank connector from B&Q. Dont get the brass one, it was too big, get the one designed for plastic pipe. Put the 22mm tank connectors onto the two tanks and then join it up with 22mm plastic pipe with inserts on each end, and voila, your tanks are connected via 22mm plastic pipe and will have good flow between them.
Ps, dont think of using copper pipe to join the tanks if you di your water before filling the tanks, apparently the di water will leach the copper from the pipes and you will end up with leaky pipes!!
Hope this helps.
Tim