I've connected two 1000lt IBC's together using flat 2" hose from a local hydraulics supplier. The hose fits over the nozzles on the IBC's and I just leave the taps open. There was a problem with the flat hose not expanding, so I put a length of 1" rigid plastic hose inside, this opens the flat hose up.
My tanks equalise in a few seconds, even though we use a BIG Draper sub pump to transfer to the vans