[Serusers] Chaining mediaproxy

How can I chain two mediaproxies? It's mentioned in the Readme, that this is possible, but it's not mentioned how. I have two SER servers in two networks, both running with mediaproxy. Client A can reach SER A, SER A connects to SER B (with forward statement), which contacts Client B. It's, because of the design of the networks, not possible that client A can connect SER B or client B can connect SER A. I configured SER and mediaproxy with default values (from the mediaproxy-example ser.cfg), however, it seems, that the two mediaproxies don't talk to each other, they are waiting for Client connection instead. Regards, Henry