First there are the Elemental Kings, as the Golden Dawn called them, or the Solar Kings as we in the O.'.S.'.D.'.L.'. called them (to avoid confusion with the Great or Supreme Elemental Kings, which continuously occurred, as well as to more oviously point out their solar nature). The name of the Elemental/Solar King is obtained from a central "whorl" of each Tablet, reading clockwise, as follows:
"Put the A or the h that stand in the Center, to it: Thou hast Bataiva or Bataivh. You must take but one of them, either the A or the h. A, comiter, and h in extremis Judiciis."A True & Faithful Relation, P. 178
Comiter is Latin for "kind", "courteus", "merciful", and so forth, while in extemis Judiciis is Latin for "in extreme judgement" (i.e. Severity). Now, apart from the obvious correlation to the Qabalah (Judgement [Din] is another name for Geburah, for example), which supports my emphatic suggestion that all students wishing to learn and use Enochian should acquire a knowledge of the Qabalah first, I have to ask: why did the Golden Dawn seemingly ignore these instructions on using only one letter? We could potentially ask if they were aware of it, but it seems hard to think that they were not, seeing as they got many other elements of name derivation correct. So why did they use both letters?
My answer to this question is that, like all other areas of Golden Dawn work, balance is key. They would never have employed just Mercy or just Severity, and so they deliberately combined both, forming a single eight-letter name, where the Elemental/Solar King would have access to both his merciful and severe aspects, allowing a multi-purpose entity, suitable for a wider range of workings, and more in line with the basic tenets of the Golden Dawn system.