If you are working an open house for another agent who is working for the same broker and someone comes in and wants to make an offer on the house would you have to become a buyers agent to get paid or would you be a sub-agent to the seller?
your broker is the "agent" of the seller, you and the listing agent represent the broker and the seller. The buyer/offeror can usually elect to also be represented by the listing broker (dual agency), if the listing broker and seller are agreeable. The buyer can elect not to be represented by the listing broker/agent. In a dual agency situation, a buyer's agent is irrelevant.
What you and the other agent are paid by your broker is a matter of office policy or specific agreement.