Sylvia Brightspear

Note that the target player searches their library (which may be affected by effects such as that of Stranglehold) and that the card they find is revealed, even though these words aren’t included in the ability’s reminder text.

To have two commanders, both must have the partner ability (featured in the Magic: The Gathering—Commander™ (2016 Edition) set) or corresponding “partner with” abilities as the game begins. A creature with a “partner with” ability can’t partner with any creature other than its designated partner. Losing a partner 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 2 generic mana 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.