Coffin cold water tank in eaves for loft conversion

Hi, Not getting much help from my builder/plumber who are too busy. Having a loft conversion done and will have 2 bathrooms in house with 3 people living there most of the time. Have gravity fed system and plan to have 2 power showers one in loft and one on first floor. What's the minimum size cold water tank I will need in eaves would it be 50 gallons as I imagine I would need 2 coffin tanks (is this possible) in that case due to height and width restrictions . Currently have a standard small HWC I think it's 120L or so. Just wandering what kind of set up I would need i.e. cold water tank and HWC capacities with a pump connected to both hot water and cold water supplies. Have decided on this set up as would rather keep existing condensing boiler as I have heard mixed reviews about combi boilers and their longevity. Also I believe megaflo type cyclinders are out of the question as it would require good mains pressure which I dont' know is adequate. Advice appreciated. Many thanks.