Jim is 100% correct, in the state of PA we can practice dual agency. A dual agency addendum must be signed and agreed upon by both the buyer and seller. I know there are many areas that do not allow this, but we have alot of success.
On the same transaction? They can, but in most states they can only be an agent for one of the parties. In Colorado we have transaction brokers, transaction brokers help tp bring the parties together, and are not an agent for the buyer or the seller.