Your program suggests a misunderstanding of the way Qpid works.

In Qpid (for AMQP protocols 0-8 to 0-10) message producers send messages to Exchanges. The Exchange is then responsible for routing the message to zero or more Queues. The precise details of that routing depend on the exchange type. It is through this mechanism that Qpid supports common messaging topologies (point-to-point, publish/subscribe, fanout etc).

Your use-case requires the use of an instance of a direct exchange (such as the built-in amq.direct).

A direct exchange routes messages to queues based on an exact match between the routing key of the message, and the binding key used to bind the queue to the exchange. A common convention is to bind the queue to the exchange using the name of queue itself as binding key. It appears your program is currently using the string "routing_key" for this purpose, and I suspect will explain the undesired behaviour you observed.

You will find more explanation here:

http://qpid.apache.org/releases/qpid-0.24/java-broker/book/Java-Broker-Concepts-Exchanges.html (Qpid Java Broker documentation - but the concept is shared)

https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_MRG/1.1/html/Messaging_User_Guide/chap-Messaging_User_Guide-Exchanges.html (Redhat's MRG docs explain the same concepts with useful diagrams)

The Python Examples (see Qpid website) are a useful reference.

You need to call open_sender to create a channel for sending messages. You can do this all in one handler. Pseudo code:

proton::sender snd;

on_connection_open(conn) {
    conn.open_receiver("queue1");
    snd = conn.open_sender("queue2");
}

on_message(dlv, msg) {
    // Relay message from queue1 to queue2
    snd.send(msg);
}

This snippet doesn't use on_sendable. The send is buffered in the library. If you prefer, you can use a vector of messages as you've done in your code and use on_sendable (in the same handler) to send when there's credit.