We can run mutiple testng suites by adding them to a suite.xml file like this

<suite name="My suite">
<listeners>
<listener class-name="CustomReport.CustomReporting" />
</listeners>
<suite-files>
<suite-file path="./Suite_A.xml" />
<suite-file path="./Suite_B.xml" />
</suite-files>
</suite>

And we can add multiple tests to a individual xml file like Suite_A.xml like this

<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd">
<suite name="Suite A">
<test name="Test A1sdd">
<classes>
<class name="testSuiteA.TestA1" ></class>
</classes>
</test>
<test name="Test A2">
<classes>
<class name="testSuiteA.TestA2" ></class>
</classes>
</test>
</suite>