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.
Donna Noble
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 Donna is paired with another creature and they are both dealt damage at the same time, the second ability triggers twice.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Andrey Kuzinskiy
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