I set the second one manually; copied the first one and added one.
vif = [ "mac=00:16:3e:5b:44:5f,bridge=pubbr", "mac=00:16:3e:5b:44:60,bridge=virtbr" ]
Looks good to me?
Interfaces are associated with both bridges properly, but only an eth0 device appears in the domU.
If the low-order bits are magic in a MAC, then maybe my second one is an invalid, but I don't find any documentation of such; the low two bits of the *first* byte have meanings, but I haven't touched them.
Well, the digits are between 0-9, A-F so I can't see it being invalid.
I guess you use Network Mangler? Since my very first experience with it, I have never installed it or used since!
jlc