Combining the Luban "multiplexer" and "AsyncPrinter" snippets and adding
some details to actually test it:  (sorry about the formatting, Usenet's
not great for that)

import java.util.concurrent.*;

public class MultiplexerDemo {
    static class Multiplexer implements Runnable {
        public void run() {
            System.out.println( "[Multiplexer starting]" );
            executor.execute( new Runnable() {
                public void run() { merge( 1, flow1 ); } } );
            executor.execute( new Runnable() {
                public void run() { merge( 2, flow2 ); } } );
            System.out.println( "[Multiplexer ending]" );
            starting = false;
        }
        public void merge( int flow, BlockingQueue input ) {
            System.out.println( "[Multiplexer merge flow" + flow +
                " starting]" );
            while ( !executor.isShutdown() ) {
                Object o = input.poll();
                if ( o != null ) merged.add( o );
                Thread.yield();
            }
            System.out.println( "[Multiplexer merge flow" + flow +
                " ending]" );
        }
        public void waitFinish() {
            while ( starting || !flow1.isEmpty() || !flow2.isEmpty() )
                Thread.yield();
        }
        protected boolean starting = true;
        public BlockingQueue flow1 = new LinkedBlockingQueue();
        public BlockingQueue flow2 = new LinkedBlockingQueue();
        public BlockingQueue merged = new LinkedBlockingQueue();
    }
    static class AsyncPrinter implements Runnable {
        public AsyncPrinter( BlockingQueue input ) {
            this.input = input;
        }
        public void run() {
            System.out.println( "[AsyncPrinter starting]" );
            while ( !executor.isShutdown() ) {
                Object obj = input.poll();
                if ( obj != null ) System.out.println( obj );
                Thread.yield();
            }
            System.out.println( "[AsyncPrinter ending]" );
        }
        public void waitFinish() {
            while ( !input.isEmpty() )
                Thread.yield();
        }
    public BlockingQueue input = new LinkedBlockingQueue();
    }
    public static void main( String[] args ) {
        System.out.println( "[main starting]" );
        Multiplexer multiplexer = new Multiplexer();
        executor.execute( multiplexer );
        AsyncPrinter printer = new AsyncPrinter( multiplexer.merged );
        executor.execute( printer );
        multiplexer.flow1.add( "Line 1 - Flow 1" );
        multiplexer.flow2.add( "Line 2 - Flow 2" );
        multiplexer.flow2.add( "Line 3 - Flow 2" );
        multiplexer.flow1.add( "Line 4 - Flow 1" );
        multiplexer.waitFinish();
        printer.waitFinish();
        executor.shutdown();
        try { executor.awaitTermination( 10, TimeUnit.SECONDS ); }
        catch ( InterruptedException e ) {}
        System.out.println( "[main ending]" );
    }
    static ExecutorService executor =
        Executors.newFixedThreadPool( 15 );
}