Other creatures you control can tap to produce enough mana to cast both creatures mentioned above after being tapped twice.
Steps
Activate any number of creatures you control by tapping them, adding an amount of mana.
Cast the creature from your hand by paying its mana cost.
Hullbreaker Horror triggers, returning another creature from the battlefield to your hand.
Resolve the creature spell. When it enters,
Intruder Alarm triggers, untapping all creatures.
Repeat.
Notes
Ideally, if the two creatures require different amounts of mana, you will want to start the combo with the lower cost creature in your hand. This should negate the need to start with mana. However, the opposite can be done if you provide mana (as needed) to cover the difference. An example of the prerequisite: If the creatures have total cost ({3} magic symbol) ({U} magic symbol) ({U} magic symbol) , other creatures you control, after tapping two times each, must produce at least ({3} magic symbol) ({U} magic symbol) ({U} magic symbol) . Therefore, if creatures you control can tap to produce at least ({2} magic symbol) ({U} magic symbol) , they will create ({4} magic symbol) ({U} magic symbol) ({U} magic symbol) after being tapped twice, and meet the prerequisite.