Text (MT): Choose two target creatures that share no creature types. Each of those creatures deals damage equal to its power to the other.
Both creatures can be controlled by the same player. [Morningtide FAQ 2008/01/15]
If, by the time Rivals' Duel resolves, an effect has caused the two target creatures to share a creature type, it is countered for having no legal targets. [Morningtide FAQ 2008/01/15]
