I don't think it would be that hard. It would be an exacting process though. You would have to decide how big the tunnel would be and then cut two holes in each of your tanks exactly in the same place on each tank. You would then have to draw out twoarches that would span the two tanks, make the top and bottom and silicone them. Once the silicone has cured, the fun part comes. Fitting the arches in each tank. If you have measured correctly, it would be a matter of siliconing them into the two tanks. Once everything is water tight, you should be able to fill one tank and as the water level rises, you might get a few air bubbles in the arch but they can be removed by forcing the water thru it. The main thing is if you two tanks you have are acrillic. If they are glass, now you have a totally new ball game.