First up I am not sure why you cannot select the second NIC but I have an alternative that may help you troubleshoot. We develop a product called LANGuardian which can connect to a SPAN port and it can also integrate with SolarWinds. You can see an example of it in action here:
If you are interesting in application timings then the SolarWinds option may be the best. If you need visibility of application and user information which includes both direct and proxied traffic then it may be worth testing out our offering.
Hope this helps
Thanks for the response. I will try and follow up the second nic option as I believe there is a cost associated with LANGuardian and with these cost saving times it is not an option. Can you please explain how I would use the monitoring nic card. The way I view it is that solarwinds (or anyone) would not be able to see the associated IP address due to the port being in monitor mode.