I have a similar setup. The only difference is that I already have a network-bridge on the real network and I added a private net as well.
First you need to create the dummy bridge on the dom0 (I call it dmzbr0):
# brctl show
bridge name bridge id STP enabled interfaces
dmzbr 8000.feffffffffff no vif1.1
And then you add new network adapters to the domU's and attach them to the bridge:
dhcp = 'dhcp'
vif = [ 'mac=00:12:34:56:78:90,bridge=eth0', 'mac=00:AB:CD:EF:01:23,bridge=dmzbr' ]
And I guess you'll handle it from here on. If not, ask