Bro ace is an excuse for people who play bad in rainbow six siege, to give them some sort of comfort.
Bro ace is when 2 people share an ace, when you split the five kills of an ace between 2 people, hence the Bro aspect of the entire thing.
Its a grand thing to happen, and must be aknowledged. You can have an ace in 1 round and still get a Bro ace in rounds after, there is also something even more rare called a squad ace, where all people get 1 kill each.
The Bro ace is a mental connection between 2 people who have each others backs, and can communicate on a different level. Getting downed will make it null and void, so the mental connection may not be broken. Protect and conquer is key.
The people who invented this are called Kebab and Daisy, which sounds like a low budget detective show. Mental bro ace people.