You gotta make sure your router has a port in the same subnet as your VM. Take a look at the Network Topology view in your OpenStack Dashboard. If there's no link from the external network via your router to the subnet with your VM you can add a port by going to "Routers", selecting the appropriate router and then clicking 'Add Interface'. Afterwards you should be able to assign a floating IP from the 2nd external network.

All this said, this only (seemed to?) worked for me for a short time. We just teared down all the virtual networks in our testbed b/c not a single VM was able to get further than the routers external IP.

Also see the bug "Multiple Floating IP Pools feature is broken" on Launchpad: https://bugs.launchpad.net/neutron/+bug/1191729

You won't be able to assign floating IP's across openstack defined networks. Have you tried adding the second subnet to your first network? (Instead of having 2 networks) Quantum will assign floating IP's from the second subnet after it has run out of IP's on the first. This will only work if the second subnet is routable through the same network.