Cultist of the Absolute

Choose a Background is a variant of the partner ability. You may have two commanders if one of them is a legendary creature with the choose a background ability and the other is a legendary Background enchantment. Backgrounds and cards with choose a Background do not interact with cards which have any other partner ability.

If your commander loses the choose a Background ability or stops being a Background during the game, as appropriate, it is still 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 combat damage from any one of them, not from both of them combined (although your Background won’t usually be a creature anyway).