Can channels of different keys be multiplexed?

No, in fact all operations are based on an instance of the PubNub object that you initialize using your pub/sub key set. Therefore, all operations invoked on that object are executed within that key set name space.

You can create another instance of PubNub using another pub/sub key set, but this is an exceptional use case and should not be implemented without considering alternate solutions. 

var pubnub1 = new PubNub({
    subscribeKey: "sub-c-1234-redacted",
    publishKey: "pub-c-1234-redacted"
});

var pubnub2 = new PubNub({
    subscribeKey: "sub-c-6789-redacted",
    publishKey: "pub-c-6789-redacted"
});

If you believe using multiple instances of PubNub object is what you require, please contact PubNub Support for further guidance on advanced design patterns, such as this.