No. An offer of compensation in the MLS becomes enforceable when the cooperating broker is the procuring cause. No other agreement is necessary.
Some brokers find the Agreement Between Brokers for Residential Leases (TAR 2002) beneficial, since it specifies a time frame for payment, and covers compensation for lease renewals and sales. Still, an agreement between brokers is not required to enforce the offer of compensation specified in the MLS.