After a player selects a Command Action and resolves it, they may immediately “Muster” a second Command Action by placing 1 Serf from their Courtyard (but not an Action Token) into an open Action Slot. Only Command Actions may be Mustered. Fully resolve each action in the order they are taken. For example, you could March, then Muster and Cast a Spell. Or even March twice (with the same or different armies) using a Muster to use both March Actions.