I Have two x 300 gallon water tanks in garage that I would like to link as backup to house pressurized system water in case of water loss in the regular water supply.
What is the most efficient way to link them.
One tank is already hooked up to the house system and serves as backup water supply.
I simply want to link second tank to double available storage.

I simply want to link second tank to double available storage.
Then you want to connect the two tanks in parallel which would give you a 600 gallon storage capacity. Good luck, Tom