We would like to share with an affiliate if the credit score is above or below 600. Would this be premissible under FCRA? If not why? Thank you

Unless you have provided an opt-out notice I think you will be violating FCRA. The FCRA exclusion at 571.3(g)(2) is for information "solely as to transactions or experiences between the consumer" and the bank. A credit score does not fit that exclusion.