Sorry, I thought you could flip coins on OCTGN in any mode, but I was wrong. To flip a coin, you can use a forum or chat service that uses it if you feel like getting that together, or use honor system. Alternatively, here is one way to do it on OCTGN.
Have ONE person load one of their decks. Have the other person guess what side they loaded. If the guess is incorrect, the first person just shows their ID and then switches to their other deck. If that person guesses correctly, then the first players keeps and plays with that deck.