<p>This article <a href="http://www.petefreitag.com/articles/gctuning/" rel="noreferrer">here</a> suggests to use <code>-XX:+UseParNewGC</code> "To enable a parallel young generation GC with the concurrent GC".  </p>

<p>My confusion is that in order to enable both parallel and concurrent GC, should I</p>

<ul>
<li>use <code>-XX:+UseParNewGC</code> or</li>
<li>use both <code>-XX:+UseParNewGC</code> <em>and</em> <code>-XX:+UseConcMarkSweepGC</code> ?</li>
</ul>

<h3>PS</h3>

<p>I am using JVM 6.</p>

Since the document you linked was for a 1.4.2 VM that's what I'll assume you're using (JVMs 5 and 6 behave differently).
From
http://java.sun.com/docs/hotspot/gc1.4.2/
if -XX:+UseConcMarkSweepGC is used on
  the command line then the flag
  UseParNewGC is also set to true if it
  is not otherwise explicitly set on the
  command line
So the answer is you only need to use -XX:+UseConcMarkSweepGC and it will enable the concurrent collector with the parallel young generation collector.
Edit: for Java 6, the same flag (-XX:+UseConcMarkSweepGC) enables the concurrent collector. The choice of collector you want depends on a few things, and you should test different configurations. But there are some very general guidelines. If you have a single processor, single thread machine then you should use the serial collector (default for some configurations, can be enabled explicitly for with -XX:+UseSerialGC). For multiprocessor machines where your workload is basically CPU bound, use the parallel collector. This is enabled by default if you use the -server flag, or you can enable it explicitly with -XX:+UseParallelGC. If you'd rather keep the GC pauses shorter at the expense of using more total CPU time for GC, and you have more than one CPU, you can use the concurrent collector (-XX:+UseConcMarkSweepGC). Note that the concurrent collector tends to require more RAM allocated to the JVM than the serial or parallel collectors for a given workload because some memory fragmentation can occur.