I have looked into this several times and I cannot find a way to enumerate and select which Nic to use. The best we can tell you is is check sequence which one was chosen.
On settings menu is a force local ip. When set xlights will use the adapter with that IP address to send out data. If that ip is not present then no data will be sent at all. This is the only operating system agnostic way I could find to solve this problem.
Worked great until I tried the scheduler. How do I accomplish the same thing in the scheduler. Can't find the comparable setting