A_DualAction is an action that performs two actions simultaneously in a single state. Var1 and Var2 specify the numbers of the states from which the actions to be performed are taken. These two states are not actually executed, only their actions are performed, so any attributes beside their Var1 and Var2 are disregarded. Although the actions are performed in the same tic, the game nevertheless executes Var1's action first internally.
By also giving the referenced states A_DualAction, it is possible to perform even more actions simultaneously. Actions can also be performed simultaneously without A_DualAction by giving the states in question a duration of 0, but this is not possible for the SpawnState.
// Description: Calls two actions. Be careful, if you reference the same state this action is called from, you can create an infinite loop.