How do you plan products that must be ordered as matched sets?

Model the set, not just the components. The requirement should be computed at the level of the sellable combination, so demand for the finished configuration drives a matched proposal across every reference in it, in the correct ratio. Each component can still have independent demand, since one half of a pair is often also sold as a spare, so the plan needs both: the set-level requirement and the component-level one, netted together.

Two safeguards matter in practice. First, any proposal that would break the ratio should surface as an exception rather than being placed quietly, because a shipment of half-sets is worse than no shipment. Second, imbalance in existing stock should be visible as its own alert, since matched-set inventory drifts out of ratio gradually and only becomes obvious when the scarce component hits zero.

In Flowlity this is handled as a constraint and grouping configuration set up during implementation rather than something planners maintain by hand.