I suppose historically the term brace comes from game angling? In those terms defining a brace is simple. Two fish in a session is a brace, simple as. That would be even more straight forward if a brace is your quota.
Gets more complicated when applied to carp because sessions often last longer than your average days trout fishing.
Unless there is in existence some form of official document that clearly defines what constitutes a brace, it is simply down to personal interpretation and ethics.
Personally I consider a brace a pair of fish caught within 12 hours of each other. Others may consider a brace a pair of fish caught within the same session irrelevant of length.
Each to their own I say, does it really matter? Unless somebody is trying to claim some sort of record, surely by stretching the bounds of credibility they are just ultimately cheating themselves? It’s a bit like a guy who would add a few ounces to make his Pb 30lb. Does it really effect anybody else? He is the only one who will hear that little voice saying “lying bast*rd” in his ear every time he boasts about his pb being 30lb.

That all said, it would be interesting to hear what others think about what constitutes a brace.