I think it falls within 'reckless' definition and Collum was spot on with the yellow. I don't think it meets the requirements for excessive force.
When the ball breaks Dunne comes from a fairly close by, he isn't full pelt from distance. The point of contact is ball initially, but when connecting with the player it is low and studs are down to ground. His second leg does make contact as a follow through but it is behind him going into the challenge.
His trailing leg comes through as you say, but I'd say a 'two footed tackle' is more one where both feet are pointing at the player and make contact pretty simultaneously.