Auras were pretty much the Marshal's whole shtick, they represented "haranguing, ordering, directing, encouraging, cajoling, or calming" his troops in battle.
You could, after 1st, have two auras going at once, a major & minor, but you could eventually know a number of each.
There really wasn't a lot of explanation of the distinction. One used CHAmod, the other a smaller scaling bonus.
I suppose you make the distinction between passive inspiration, lead by example, keying if CHA, requiring allies be able to see you, and active tactical direction, requiring subordinates be able to hear & understand your orders, keying off INT. That'd be a rationale for having em both up at once.