I think you can achieve this in two ways:
1) Instead of adding the second class to the TestNG.xml, simply import it into the first test class and invoke its method (add customer details) from the first class @test method. The advantage here is that you can easily check if the login succeeds or not, decide to call the 2nd class method or not and even pass some data as parameters.
2) Mention two classes as a part of the single test cases, check below:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd">
<suite name="Simple Test Suite" verbose="1">
<test name="Testing Part1">
<class name="com.techbeamers.testng.LoginPage" />
<class name="com.techbeamers.testng.addCustomerDetails" />
Here, the test methods from each class will get executed one after the other. And it may get fulfilled what you wish to achieve.