Might be some miscomunication there. It acts as a lever arm. The tank is next to the wall, so that is where most of the load is, not at the posts 10' away. Like a lever, you need to beef it up near the fulcrum. The fulcrum being at the wall where it goes to the foundation. So yes, you need to make sure you SOLIDLY support the sister joists down to the foundation, at the wall. But you only really need to do that end, not the other end. And as you get farther from tank, you have less load, hence being able to cheat a bit, and not go the whole length. And not going the whole length simplifies installation hugely.