In practice the pairing is dramatic rather than doctrinal. At midwinter the Oak King overcomes the Holly King and the light begins to return; at midsummer the reverse. Groups that use it stage the meeting in various ways, from a spoken exchange between two people in seasonal dress to a stylised combat, and the point being made is about the turning rather than about either figure winning. Some traditions treat the two as one being seen from two sides, which is the form closest to Graves's own tanist argument.
The framework does something the eightfold calendar otherwise lacks. The solstices are astronomical facts with no agricultural work attached, unlike the harvests and the sowings, and the two kings give them a story.
Behind them lies a real body of custom that should not be confused with them. Seasonal combat and revival is a genuine feature of English folk drama: the mummers' play, performed at midwinter across many regions, turns on a fight, a death and a doctor who raises the fallen man. Holly and ivy appear paired in carols and in midwinter greenery, sometimes in explicitly competitive terms. Those customs are documented and old, and the folklorists who once read them as survivals of pre-Christian ritual are now generally regarded as having overreached.
The honest description is therefore that the Oak King and the Holly King are a modern figure built from an argument about poetry, working alongside older customs they resemble but do not descend from. Many practitioners say so themselves, and hold that a well made observance does not need to be ancient to work.