Do it even easier:
Run acme package on FW1 (I assume it's a CARP cluster with syncing?) and let it create a certificate for both names (fw1.xxx AND fw2.xxx). When it's done, select the cert for the webui. Then login to FW2 and select it, too, as certificates get synchronized automatically (if selected) to the secondary. There choose the same certificate as WebUI cert and be done :)
Just check that you configure the acme service on fw1 to restart its own webserver after renewal AND via remote the service on fw2 (see the help for this)!
Greets