Yes it is. I've tried it today but there are a few issues.
1. Do not use AVB network on the second mixer. It will 'steal' your talkback port from the first mixer.
2. If you press solo on your second mixer, it will also set the first mixer on solo. It is also useless, because the headphones won't work if you are not on a AVB connection.
We tried this with 2 RM32AI devices connected.