Ian,
Did your IBC's come with spouts on the bottom taps? I've got two joined together by a length of flat (fire hose type) hose, pushed over the spouts on each IBC and held with a couple of big hose clips. I leave the taps open so there is a flow between the two tanks. I put a length of 2" rigid plastic hose inside the flat stuff to stop it collapsing and closing the flow. I have a sub pump in one of the tanks, so as we draw water from that tank the level balances with water flowing in from the other tank.