The jabsco is designed specifically as a reversible pump. Thus, it will fill and drain in almost equal time reguardless of the fill location.
All boat manufacturers that use metal ballast reversible pumps fill and drain from the same sac/hard-tank valve. I think that the only way you could possibly do what you are looking to do would be through the use of a check valve; however, the check valve will slow the flow of water and that is exactly what you are trying to avoid.
In closing, fill and drain your sac from the (1) valve that is closest to the rear of the boat and closest to the floor. While your vent valve should be on the top of your sac closest to the front of the boat. This is the best way to fill your sac with the one pump you already own.
You could buy another jabsco and do your fill from the top and have the second pump only drain, but you are defeating the purpose of a reversible pump and spending money that you don't need to be spending.
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