In our Teamcommander Rules, we stated that we are following the official free for all Commander Rules for Deckbuilding as they are here:

Deck Construction Rules

1. Players choose a legendary creature as the commander for their deck. 
2. A card’s colour identity is its color plus the colour of any mana symbols in the card’s rules text. A card’s color identity is established before the game begins, and cannot be changed by game effects. The cards in a deck may not have any colours in their color identity which are not in the colour identity of the deck’s commander.
3. A Commander deck must contain exactly 100 cards, including the commander. If you’re playing a companion, it must adhere to color identity and singleton rules. While it is not part of the deck, it is effectively a 101st card.
4. With the exception of basic lands, no two cards in the deck may have the same English name. Some cards (e.g. Relentless Rats) may have rules text that overrides this restriction.

We received some Questions about this, and we want to clarify our Rules; while we THINK that the official rules do mean the same as we do, they don’t state it directly.

1. Players chhose a legendary creature, a legendary Planeswalker, which states that you may play him/her as a Commander, or two legendary creatures with the according, matching partner ability for their Deck.
2. – unchanged –
3. A Commander deck must contain exactly 100 cards, including the
   commander (or the commanders if using partner commanders). If you’re playing a companion, it must adhere to color
   identity and singleton rules. While it is not part of the deck, it is
   effectively a 101st card.
   
4. – unchanged –