Using all of the dechorinator in the first load would mean adding enough for the tank volume, instead of using the amount for the water change. Because dechlorinator can be used trying to neutralize the organics in the water, dosing that is not done in the new water needs to account for the entire tank volume. I find it easier to just dose a bucket at a time for my water changes. Place the bucket in the sink and start filling it. When I have an inch or two in the bucket, add measured dechlorinator for the bucket full. When the bucket is full, the dechlor has been well mixed so go dump the water into the tank. In my case I use a siphon to add the water to the tank just to limit the amount of tank water stirring I do with my water change. I hate it when there are things floating around in my water that don't belong there.
