**A water-filled siphon over the tank rims can equalize the levels without drilling the tanks.** Unlike a temporary transfer, it can stay in place and resume flowing whenever one tank’s water surface becomes higher than the other. This uses the principle demonstrated by [Iowa State University’s siphon experiment](https://learning.physics.iastate.edu/siphon).

With the owner’s agreement:

1. Run an airtight, drinking-water-rated hose or pipe over the rims, with one end near the bottom of each tank.
2. Fill it completely with water, removing all air bubbles.
3. Keep both ends closed while positioning them underwater, then open them. Prime it by filling or using a suitable pump, rather than sucking by mouth.

Water will flow from the higher water surface toward the lower one and stop when their elevations match, assuming both tanks are vented to the atmosphere. **Equal elevation means equal depth only if the tank bottoms are at the same elevation.**

There are two practical limitations:

- Both ends must remain submerged. If either tank exposes an end, or air enters or accumulates in the pipe, the siphon can fail and need priming again.
- During filling or use, a small level difference is necessary to drive flow. A narrow hose may not transfer water fast enough to keep the levels close.

Being on a three-storey roof does not change this principle; the relevant height is the pipe’s crest above the water surfaces.

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