Both units are configured with static IP addresses and both units are connected to the MOTU AVB switch.
It's best to have a wifi router connected to the AVB switch and leave both mixers in Dynamic mode, so they get an IP address from the router, and if you connect any computers or other remote devices, they'll get an IP address from the router as well. Without the router, you'll have to configure and manage IP addresses for every device involved. The wifi router will also give you the ability to add wireless control devices.
I can select one of the mixers in UC but never find the second mixer, or at least the correct IP address.
This sounds like you have a network/IP configuration issue, with either mixer, or with your computer, which again is why it would be recommended to use a wifi router and leave the mixers in the default Dynamic mode, so that everything automatically gets an IP address from the router.
See here for more info.
