In your 'foreach' loop, you can create a 2nd array of Network adapters.
$myVmNetworkAdapters = Get-VM $vm | Get-NetworkAdapter | select NetworkName, Parent
And in theory you could write an if statement. You have the option to set additional variable names based on the index in the array or just specify the index in the if statement.
If ($myVmNetworkAdapters[2].NetworkName = $myVmNetworkAdapters[3].NetworkName) { Do something (not sure what you want to do here) }
Now I didn't test the syntax, but hopefully you have the idea.