Sharing separate WAN connection to another customer

Hi all,
We have a client with a z4 plus in Building A and well as a fiber circuit with two available WAN IPs. The client owns a neighboring building (building B), and rents it out to another company.
The Building A is attempting to off-set his monthly fiber service costs by creating a building to building bridge (https://store.ui.com/products/unifi-...uilding-bridge) with building B. Building B has their own Sophos firewall, and would like to continue using it.

What is the best way to achieve sharing the separate WAN IP between the buildings? Ideally, I'd like to utilize untangle in Building A to provide the WAN connection to building B in order to prevent building B from saturating the connection with high load, but I'm not sure how to achieve this.

Thanks for your help.

I would use a separate NIC on the z4 to building B. Bridge the NIC to the WAN (DMZ mode). Then use QoS and Bandwidth Monitor to set allowable bandwidth.