The triggered ability triggers both when Bruse Tarl enters the battlefield and whenever it attacks. You don't have to choose only one.

If your Commander deck has two commanders, you can only include cards whose own color identities are also found in your commanders' combined color identities. If Akiri and Silas Renn are your commanders, your deck may contain cards with white, blue, black, and red in their color identity, but not green.

Both commanders start in the command zone, and the remaining 98 cards of your deck are shuffled to become your library.

To have two commanders, both must have the partner ability as the game begins. Losing the ability during the game doesn't cause either to cease to be your commander.

Once the game begins, your two commanders are tracked separately. If you cast one, you won't have to pay an additional the first time you cast the other. A player loses the game after having been dealt 21 damage from one of them, not from both of them combined. Command Beacon's effect puts one into your hand from the command zone, not both.

An effect that checks whether you control your commander is satisfied if you control one or both of your two commanders.