NFPA 22 is the standard for firewater tanks, however I don't think you will find any minimum distance between two tanks, other than adequate clearance for inspection and maintenance activities.
Separation of redundant firewater supplies (storage and pumping), e.g. at a large process plant with fire / explosion hazards should be based on a risk assessment so they are not subject to the same credible fire / explosion scenario. I thought I had a recommended separation distance for this item, but can't find it, typically at two hydraulically remote opposite parts of the plant firewater system.