The relationship you posted is not possible in FileMaker.
This is what you should have:
Transaction::_k_seller1_id = Customer::_p_customer_id
Transaction::_k_seller2_id = Customer 2::_p_customer_id
Where Customer and Customer 2 represent different Table Occurrence "boxes" in your relationship graph.
You can then define two calculation fields in Transaction that look like this:
Customer::LastName & ", " & Customer::FirstName
Customer 2::LastName & ", " & Customer 2::FirstName
A simpler approach would be to define a single calculation field, cFullName, in the Customer table as:
LastName & ", " & FirstName
and then add the fields Customer::cFullName and Customer 2::cFullName to your Transaction layout.
If Table Occurrence is new term for you, you might want to read this thread: Tutorial: What are Table Occurrences?