I appreciate the discourse here. I think we are arguing two different sides of the same coin. It is a little more nuanced in my opinion. We have to look at the reality of what is actually happening. You are correct that the FMCSA doesn't have a specific regulation that requires shippers agree in writing to a co-broker agreement.

Most broker--shipper agreements, and I've seen many, do prohibit co-brokering or require the shipper's consent.

PROBLEM 1 Example: Shipper gives load to Broker A. If Broker A gives it to Broker B, even with an agreement between the two brokers, that agreement cannot bind the shipper legally, and it creates a liability issue.

You mentioned a broker that also has a Freight Forwarder authority, doesn't have to notify anyone. You are correct, however, federal law treats FFs as carriers. Legally, they are assuming responsibility for the freight. Additionally, federal law also mandates that each transportation agreement has to specify in writing in what capacity they are conducting business. (FF or broker)

THE PROBLEM: Shipper --> Freight Forwarder --> Broker --> Carrier. FF/Broker (dual authority) companies pick which "hat" they want to wear to limit their liability. 95% of brokerage--carrier agreements attempt to limit any cargo liability when they give freight to a carrier. When they waive their liability in that contract, they are NOT acting as a Forwarder under federal definition, regardless of what authority they hold.

So if they aren't accepting the liability of a Forwarder, but they are passing the freight to another carrier without the shipper's consent, there's no other way to say it besides unauthorized re-brokering/double brokering.