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There's 2 different kinds - helper springs, and secondary springs.
A helper springs purpose is to keep the "real" spring seated properly which reduces noise. Sounds like that's what the both the Tanabe and Tein HA uses. I would guess the Tein SS doesn't need helper springs because the spring rates aren't as extreme as the HA, and the main spring is long enough to not need them.
Secondary springs are stronger and contribute to the ride characteristics of the suspension - in essence, they'll use two different linear spring rates which act together to create a progressive setup. Those setups aren't terribly common, though. If a company wants a progressive acting coil-over, it just makes more sense to wind a single progressive spring. Although a 2-spring setup has the advantage of being user-tunable by swapping around either of the springs.
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