Dan,
Your question seems to indicate that you want to run two Flow services in parallel each on its own processing thread. You have to write a java service or two to do this. The Service.doThreadedInvoke method allows you to specify a service name to invoke and the IData containing the inputs of that service. See the WmSamples package for one example. You can also search WM Users for doThreadedInvoke to turn up more discussion on this point.
Integration Server is multithreaded. Using multiple threads to perform your integration work is a process of analyzing the work to determine what can be executed in parallel, invoking multiple additional threads to do the work and then collecting and reassembling the results (including any exceptions).
Mark
#webMethods#Flow-and-Java-services#Integration-Server-and-ESB