We have the same issue and called solarwinds to try and work through this. The response was..
"We spoke about this and monitoring from Orion in a multi-homed environment is dependent on which nic is the primary nic in the network bindings. While the network bindings are configurable in Windows there can only be one primary. Orion then uses this primary nic for polling. From my own experience and from discussing with the team this is the limitation. From the team this limitation comes from the OS that it presents one nic we must use that nic and also we expect traffic back on that nic. As requested I'll submit a feature request for this functionality."
I plan on posting this feature request in the forums later on. Hope this helps you,