A player can have two commanders if both have partner. Because both commanders start the game in the command zone, the remaining library is only 98 cards. Your two commanders are treated individually for all other Commander rules. If either leaves the battlefield, you can return it to the command zone instead. The "commander tax", the additional cost of 2 for every time you've cast a commander from the command zone, applies individually. Combat damage dealt by each commander is similarly tracked individually. If either commander can deal 21 or more combat damage to a single player over the course of the game, that player loses (Fandom).
