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My guess is that the larger tank is an accumulator tank for your on board fresh water. This provides storage to reduce pump cycles and cushions the hammer from the pump starting and stopping.

The smaller one is probably for thermal expansion for the water heaters. Most systems do not require a thermal expansion tank because the pressure can expand into the cold water supply and accumulator. If a check valve is installed in the water heaters cold water feed line, the pressure will not be able to expand back into the cold water supply line and accumulator tank resulting in the saftey valve opening. This is usually why a thermal expansion tank is needed. Jeff