It is possible to attach multiple vNICs to an instance, there is however currently a limitation that means these vNICs must be on separate subnets though. Here is an example from the documentation:

# $ nova boot --image ed8b2a37-5535-4a5f-a615-443513036d71 \
              --flavor 1 \
              --nic net-id= <id of first network> \
              --nic net-id= <id of first network>  test-vm1

More information is available here: http://docs.openstack.org/admin-guide...

The bug discussing the limitations I note above is here: https://bugs.launchpad.net/nova/+bug/...