I used a syphon tube (water finds its own level). The tricky bit bit was to fill it and put it in the other 'roll before it leaked out. In the end i used 2 lengths of black rigid electricians conduit cut at an angle. being rigid they sit nicely at the bottom of the rolls and cut at an angle so with the "point" sat on the bottom the water can still flow up / down. I connected a couple of small (3 inch) lengths of clear flexible water pipe to these and a "Y" connector to join them together. I then fixed a wine makers syphon tap into the single end of the "Y" connector. All you do then is pop one leg into each aquaroll, open the tap, suck up the water, put your tongu over the end and close the tap. Bingo!. 80 litres. Only drawback i found was having to fill up both rolls at the same time.
If you really want to go whole hog fit a water level gauge to one of the rolls.