KVM second private network (bridged), how to get working?

what you need to do are a few things.

  1. do NOT provide the NIC itself with an IP, the bridge should have that IP
  2. make sure your routing tables are set right
  3. if the guests are attached to both networks, make sure they get two virtual NICs that are plugged into the public and private bridges respectively
  4. make sure the routing table in the VMs is correct.

The networking setup scheme should be as follows:
eth0(no IP)->br0(with public IP)<-VM(public IP)
eth1(no IP)->br1(with private IP)<-VM(private IP)