Splinter, the Mentor

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 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 your Commander deck has two commanders, you can only include cards whose own color identities are also found in your commanders' combined color identities. If Donatello, the Brains and Splinter, the Mentor are your commanders, your deck may contain cards with blue and/or black in their color identity, but not cards with red, green, or white. (If Leonardo, the Balance is one of your commanders, your deck may contain cards with any color or combination of colors in their color identity, regardless of who your other commander is.)

Mutagen tokens are a kind of predefined token. Each one is a colorless artifact with the artifact subtype Mutagen and the ability "{1}, {T}, Sacrifice this token: Put a +1/+1 counter on target creature. Activate only as a sorcery."

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 damage from any one of them, not from both of them combined.

Some spells and abilities that create Mutagen tokens require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won't resolve. The Mutagen token won't be created.

To have two commanders, both must have appropriate partner abilities as the game begins. Losing the ability during the game doesn't cause either to cease to be your commander.

You can have two commanders if each one has "Partner—Character select." You can't mix and match them with cards that have other partner abilities.