It really depends on the horse.
Our little Holly reacts to stress (and she can be a bit of a drama queen so her idea of "stress" is anything she doesn't like, lol) with loose poo. For her, I'm not usually too concerned unless it goes on for a length of time I am uncomfortable with or she shows other signs that concern me (going off feed, being listless, seeming off in any way). Our vet is in total agreement with this assessment.
Aero, on the other hand, would be more of a concern to me were this to start with her.
Has there been a change in their diet recently? Have you taken them anywhere out of the norm where they may have eaten something new/different? Are they acting off at all?