To Run Multiple suites using TestNG XML the correct code goes below, Where I have prepared Three suites suiteA.xml, suiteB.xml, suiteC.xml and have consolidated them in testng.xml. You can copy paste the below code and change the packagename.classname in the class tag and run it would work...
suiteA.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd" >
<suite name="SuiteA" >
<!-- suite name="Suite Name" -->
<test name="TestA1" allow-return-values="true">
<classes>
<!-- packagename.Testcase class name -->
<class name ="com.qtpselenium.suiteA.TestCaseA1" />
</classes>
</test>
<test name="TestA2" allow-return-values="true">
<classes>
<!-- packagename.Testcase class name -->
<class name ="com.qtpselenium.suiteA.TestCaseA1" />
</classes>
</test>
</suite>
suiteB.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd" >
<suite name="SuiteB" >
<!-- suite name="Suite Name" -->
<test name="TestB1" allow-return-values="true">
<classes>
<!-- packagename.Testcase class name -->
<class name ="com.qtpselenium.suiteB.TestCaseB1" />
</classes>
</test>
<test name="TestB2" allow-return-values="true">
<classes>
<!-- packagename.Testcase class name -->
<class name ="com.qtpselenium.suiteB.TestCaseB2" />
</classes>
</test>
</suite>
suiteC.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd" >
<suite name="SuiteC" >
<!-- suite name="Suite Name" -->
<test name="TestC1" allow-return-values="true">
<classes>
<!-- packagename.Testcase class name -->
<class name ="com.qtpselenium.suiteC.TestCaseC1" />
</classes>
</test>
<test name="TestC2" allow-return-values="true">
<classes>
<!-- packagename.Testcase class name -->
<class name ="com.qtpselenium.suiteC.TestCaseC2" />
</classes>
</test>
</suite>
testng.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd" >
<suite name="TestNG Dadadriver suite" >
<!-- suite name="Suite Name" -->
<suite-files>
<suite-file path="./suiteA.xml" />
<suite-file path="./suiteB.xml" />
<suite-file path="./suiteC.xml" />
</suite-files>
</suite>