title: how to merge two RDD to one RDD
body: <p>Help ,I have two RDDs, i want to merge to one RDD.This is my code.</p>
  
  <pre><code>val us1 = sc.parallelize(Array(("3L"), ("7L"),("5L"),("2L")))
  val us2 = sc.parallelize(Array(("432L"), ("7123L"),("513L"),("1312L")))
  </code></pre>