Legions operating together in Britain?

[Nathan Ross]

There was a balancing legionary detachment at Carlisle too

Good god, man, next you'll be telling us there were legionaries on Hadrian's Wall!... hock:


These double-legion detachments seem to turn up quite a bit elsewhere, don't they, particularly at the end of the third century: mobile vexillations formed of men of two legions from a single province. Why the Romans should do it this way I don't know - surely using men from a single legion would be easier? Or would that just be too straightforward?