ビル・ポッツ

あなたがビル・ポッツのみを対象としてインスタントやソーサリーである呪文１つを唱えるかビル・ポッツのみを対象として能力１つを起動するたび、その呪文や能力をコピーする。そのコピーの新しい対象を選んでもよい。この能力は、毎ターン１回しか誘発しない。
ドクターのコンパニオン

Bill Potts

Whenever you cast an instant or sorcery spell that targets only Bill Potts or activate an ability that targets only Bill Potts, copy that spell or ability. You may choose new targets for the copy. This ability triggers only once each turn.
Doctor's companion (You can have two commanders if the other is the Doctor.)

Doctor Who

Although Doctor's companion is a new variant of the partner ability, the rules for partner have not otherwise changed. Notably, Time Lord Doctors and cards with Doctor's companion do not interact with cards which have another partner ability.

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

Both commanders start in the command zone, and the remaining 98 cards (or 58 cards in a Commander Draft game) of your deck are shuffled to become your library.

If something refers to your commander while you have two commanders, it refers to one of them of your choice. If you are instructed to perform an action on your commander (e.g. put it from the command zone into your hand due to Command Beacon), you choose one of your commanders at the time the effect happens.

If the spell or ability that's copied has an X whose value was determined as it was cast, the copy will have the same value of X.

If the spell or ability that's copied is modal (that is, it says "Choose one —" or the like), the copy will have the same mode or modes. You can't choose different ones.

If your Commander deck has two commanders, you can include only cards whose own color identities are also found in your commanders' combined color identities.

Once the game begins, your two commanders are tracked separately. If you cast one, you won't have to pay an additional {2} 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.

The Doctor's companion ability allows you to have two commanders if one has the ability and the other is a legendary creature that is a Time Lord Doctor and has no other creature types. Creatures with the changeling ability, for example, can't be a second commander this way.

The copy made by Bill's triggered ability is created on the stack, so it's not "cast" or "activated."

The copy will have the same targets as the spell or ability it's copying unless you choose new ones. You may change any number of the targets, including all of them or none of them. The new targets must be legal.

The first ability of Bill Potts will trigger as long as it is the only target of that spell or ability, even if that spell or ability could target multiple permanents.

You can't choose to pay any additional costs for the copy. However, effects based on any additional costs that were paid for the original spell are copied as though those same costs were paid for the copy too.