Upvote:1
From Rzepka, Jacek. "The units of Alexander’s army and the district divisions of late Argead Macedonia." Greek, Roman, and Byzantine Studies 48.1 (2008): 39-56 comes a possible answer.
On p49
It is the interpretatio difficilior, but (with Curtius’ variant in mind) I suspect that Arrian’s τελευταία δὲ τῶν βασιλικῶν ἰλῶν reflects a onetime dividing of the ile basilike into two units, a dividing that did not establish a new structure, but was determined by tactical considerations.
And
Therefore, I suspect that there were six units of the “territorial” Companions and an ile basilike of double the strength of a normal ile. At the beginning of the anabasis these seven (or eight) units together with prodromoi numbered 1800 men (Diod. 17.17.4).
Some pages earlier, the author Rzepka admits that his interpretation can be challenged since the plural of royal squadron is only used once.
On p47
In the Greek for this translation, or rather paraphrase, Brunt (LCL) bracketed βασιλικῶν, certainly because he knew that there was only one ile basilike, the one commanded by Cleitus. Admittedly, this passage of Arrian is, so far as I know, the only occurrence in Classical literature of ilai basilikai in the plural. Of course, one could try to explain that all units of Companion Cavalry and Companion Infantry must have been Royal (and were non-technically called Royal),19 and only a few were King’s Guards sensu stricto with their role stressed by official names.
As conclusion: The fact that that Arrian speaks of two royal squadrons was probably caused by a temporary separation of the unit for tactical reasons, since it was double the strength of a regular unit.