The two armies in the Imperial Presence late 4th C

[Nathan Ross]

the old Empire border in the West is fairly 'short'... Having 'the' Praesental Army split into two might make perfect sense for just that reason.

Hmm, I suppose so - although the Rhine and the upper Danube together is a frontier of around 1250 miles. The lower Danube section in the eastern empire is only about 700 miles. Then there's the eastern frontier to consider as well, of course.

But the Praesental armies were not intended for frontier defence. The magistri militum in Gaul, Illyricum and Oriens had their own armies to handle any defence-in-depth requirements. I suppose you could argue that one of the two praesental armies in the east was for expeditions towards the Danube, and the other towards the east. But if an emperor was mounting such an expedition he'd presumably want to bring all the troops available to him, which would surely mean both armies.

If you add up the troops of the two eastern Magistri Militum in Praesentalis, you get a very similar number of units of various sorts as the central (Italian) command of the single western Magister Peditum. That implies that the two eastern armies were originally one army, the same size as the western one.