The situation: I am playing MTGO and I have a deck with Keruga, the Macrosage in my sideboard and Response // Resurgence in my main deck.
Keruga, the Macrosage states the following, emphases mine:
Companion — Your starting deck contains only cards with converted mana cost 3 or greater and land cards. (If this card is your chosen companion, you may cast it once from outside the game.)
When Keruga, the Macrosage enters the battlefield, draw a card for each other permanent you control with converted mana cost 3 or greater.
The problem: MTGO shows that Keruga, the Macrosage is not a valid Companion once Response // Resurgence is included in my deck, a card with a CMC of 7.
Referencing both rules of split cards:
708.4. In every zone except the stack, the characteristics of a split card are those of its two halves combined. This is a change from previous rules.
708.4b The mana cost of a split card is the combined mana costs of its two halves. A split card’s colors and converted mana cost are determined from its combined mana cost. An effect that refers specifically to the symbols in a split card’s mana cost sees the separate symbols rather than the whole mana cost.
So it should be allowed to cast Keruga, the Macrosage as a companion in this case, correct?