I'm pretty certain a listing agent would not have an outside agent (any agent) from another firm hold an 'open house' for their listing. Sometimes another agent from the same office will volunteer to do this if the listing agent has several open houses on the same day, as mentioned by Jeannette.
If you are assuming that all agents are divided into one type or another - buyers agent or listing agent - this is rarely the case. Most of us will work either side of a transaction, but not necessarily on the same transaction. Some do though.
The listing contract is between seller and broker. When an agent other than the listing agent does the public open house, the broker has a fiduciary to the seller to represent that property. Whether I do an open house for my seller or another agent in my office, my first obligation is to promote that property. Sure, an agent is interested in buiding business and finding clients. The agents desire to build a client base needs to be secondary to promoting the open house for sale. If it becomes clear that property does not meet the needs of the open house visitor, then the open house host can take on the role of a buyer's agent.
Other than that the policy is remember to keep it legal moral and ethical.
I don't see agents from broker A's firm holding open houses for an agent from Broker B's firm. It happens but not to often.
Usually - the agent who holds the listing does the actual Open House advertising. As an agent who has conducted open houses for my colleagues - I don't like to leave things to chance. I am free to "advertise" to as many potential buyers or other interested parties.
The name of the game is to sell the house. Therefore - all is open at an open house. But I dont get your last sentence. Did you miss something?