A water-filled siphon over the tank rims can equalize the water levels without drilling the tanks. It can stay in place and resume flowing whenever one tank’s water surface rises above the other. [Iowa State University’s siphon experiment](https://learning.physics.iastate.edu/siphon) demonstrates the principle.

With the owner’s agreement:

1. Run an airtight hose or pipe rated for drinking water over the rims, with one end near the bottom of each tank.
2. Fill it completely with water and remove all air bubbles. Use filling or a suitable pump to prime it; do not suck by mouth.
3. Keep both ends closed while placing them underwater, then open them.

If both tanks are vented to the atmosphere, water will flow from the higher surface to the lower one and stop when the surfaces reach the same elevation. The water depths will be equal only if the tank bottoms are also at the same elevation.

Both ends must stay submerged. If either end becomes exposed, or air enters or collects in the pipe, the siphon may stop working and need priming again.

During filling or use, a small difference in water level is needed to drive flow. A narrow hose may not move water fast enough to keep the levels close.

The tanks being on a three-storey roof does not change the principle. The relevant height is how far the pipe’s crest sits above the water surfaces.

Your proposed bottom connection is better for dependable long-term operation. For drinking water, have a plumber assess the connection and provide backflow protection, since connecting storage systems can create routes for contamination. See [EPA’s guidance](https://www.epa.gov/dwcapacity/distribution-resources-small-drinking-water-systems).