"I want to list the contact information right on the TRANSACTION FORM but when you do this there is no way to list both agents information because Filemaker does not know which field belongs to _kf_listingagent_id vs. _kf_sellingagent_id within the TRANSACTION FORM."
Filemaker can list info for both agents if you establish two different relationships to AGENT from Transaction.
Transaction::_kf_listingagent_id = Agent::AgentID
Transacttion::_kf_sellingagent_id = SellingAgent::AgentID
SellingAgent is an added table occurrence of Agent. To create it, open Manage | Database | Relationships, click AGENT to select it and then click the button with two green plus signs. You can double click this new occurrence to change its name.
Now you can add contact fields from Agent for your listing agent contact fields and fields from SellingAgent for your selling agent contact fields.
If "Table Occurrence" is a new concept, you might check out this link for more info: