If the second improper action were allowed, or prompted by the dealer, that too would be considered substantial action, would it not? If not, then we must erase substantial action from all head to head action. I believe the dealer counts in the equation.
Guillaume Gleize,
I like your definition of two actions involving chips or any three actions, I especially liked when you used "Freeze the action." All you have to do is consider whether you want to include the dealer as one of those participants. If it is too confusing, leave it the way you have it. There are always other factors that the floor needs to consider, and when we are talking about substantial action (or significant action), it involves premature betting, out of turn betting, and too many situations to mention.