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If you open the model you see that the only connection between the heat exchanger and the volume is through the heat port of the volume. I therefore think you should be able to change Storage.StratifiedEnhancedInternalHex to have more than one instance of the heat exchanger indTanHex.
The arrangement that you show does not represent the right physics as it does not allow buoyancy from tan1 to tan or heat conduction between the fluids of these two tanks.