Could negative pressure occur in parallel unvented cylinders?

Question from Alan Hyatt, via email:
If I connected two unvented cylinders in parallel would negative pressure occur in one of them if the inlet was turned off?

Or put another way, could induced syphonage occur with the active cylinder pulling water from its parallel partner – resulting in lower atmospheric pressure within and consequential collapse of the cylinder with its inlet (mains) no longer pressurising it?

Answer from Martyn Bridges, director of marketing and technical support at Worcester, Bosch Group:
If the cylinders have been piped in such a way that the cold mains connection to both cylinders are separate, yet obviously from the same source, and the hot water outlets from the cylinders are combined, then the potential can’t occur.

That is if the mains water fails then neither cylinder will receive water, so neither can siphon water from each other.