Some sort of thingy which applies load and allows you to measure deflection. I think I once saw such a device specifically for doing this job, possibly from one of the suppliers of set-up stations, fancy tools etc. Imagine a column that sits between and in-line with the springs. At 90 deg to this is an arm that sits across the top of the 2 springs. The column is threaded, and as you tighten it up it pulls the arm which compresses the two springs. The arm is free to pivot, revealing which spring is resisting the compression most.
I might have made the above up.
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Nahh, it's real.