You cannot create multiple VMSwitches using the same physical (or teamed) network adapter. You even get a message saying you can't because the adapter is already bound to another VMSwitch.So if you want to use multiple networks you would need to setup VLANs on your physical switch, and use VLAN tagging on the vNIC or inside the guest OS to be able to work with multiple networks.
Though that means you can only use VM's on the same Hyper-V host to connect to and through the OPNsense VM for internet connectivity.
franco, there are no issues on FreeBSD 10.1/pfSense, but with FreeBSD 10.2/OPNsense port forwarding is not working and my routing table being reset after any config change from the web UI (i'm setting upstream gateway for wan interface manually).