Does Throughput calculated adds up using multiple (all active) physical adaptor

From the Netperf Manual:

3. The Design of Netperf

Netperf is designed around a basic client-server model. – –

Once the control connection is up and the configuration information has been passed, a separate “data” connection will be opened for the measurement itself using the API's and protocols appropriate for the specified test.

As it is testing the throughput between those client and server it depends on that connection. In order to test the two adapters together they should be load balanced in a way that utilizes both adapters for a single connection. That would probably not be a very useful configuration for anything else but this benchmarking.