Ravi you are right, we need to two agents for acheiving this.
Other worse / Crazy ideas -
1) Create an agent with Pattern as periodic and set the interval to 43200. Now timig is important, exactly at 12 noon, create this agent and you are done. This agent would run at 12 noon and exactly after and exactly after 12 hours. :(
2) Run agent activity periodically for an hour and in agent activity check the time stamp, if time stamp is 12 noon or 12 night then excute the activity else exit.