I need to credit 2Payees - 1 assigned to the transaction and 1 mentioned in one of the GA's of the transaction
Can you share some screen shots of the Credit rule where you are comparing Payee to the GA? What Dan is suggesting should work. (Be sure that in the rule with the comparison you do NOT have the check box checked for They are identified on the Transaction)
However, something to consider is the volume of Transactions x Payees in the system. If you have large volumes of Transactions and/or Payees a string comparison in the Condition of the rule can have performance impacts.
As Jon Mentions, the best option is to have both Payees assigned to the Transaction (1 Transaction, 2 assignments), as this is the most efficient way to allocate Credits. Are you able to put both Payee's on an assignment for the transaction?