Multiple runTest tags can be used to mention multiple test classes in RunSpecifiedTests in ANT in Salesforce Metadata Migration.

Sample code in Build.xml:

 <target name="deployProdSpecificTest">
  <sf:deploy username="${sf.produsername}" 
  password="${sf.prodpassword}"
  serverurl="${sf.serverurl}"
  deployroot="Retrieve" 
  testLevel="RunSpecifiedTests">
  <runTest>DS_CasePages_Test</runTest>
  <runTest>DS_CustomerDetailsNewCtrlrTest</runTest>
  </sf:deploy>
 </target>