Parallel execution on Test in TestNG is opening separate browser for each class in the test, it is not executing one after another

Question

Parallel execution on Test in TestNG is opening separate browser for each class in the test, it is not executing one after another.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd">
<suite name="Project1" parallel="tests" thread-count="2">
<listeners>
    <listener class-name="automation.listeners.ExecutionListener"></listener>
</listeners>
<test name="ChromeBrowser">
    <parameter name="browser" value="chrome"></parameter>
    <classes>
        <class name="test.Test1"></class>
        <class name="test.Test2"></class>
    </classes>
</test>
<test name="FirefoxBrowser">
    <parameter name="browser" value="firefox"></parameter>
    <classes>
        <class name="test.Test1"></class>
        <class name="test.Test2"></class>
    </classes>
</test>

The above TestNG.xml file is opening 2 chrome browsers and 2 Firefox browsers at once and it is executing them. I want is 1 chrome and 1 Firefox browser should be running at once. In Chrome browser, Test1 should execute and then Test2 should start execute but this is not the case.

Can anyone help me with this?

TestNG version 7.0.0

Example.class

@Parameters({"browser"})
@BeforeClass
public void setup(String browser){
    driver = /* assigning the driver w.r.t given browser */
}

@Test
public void test1(){
    /* My code */
}

@Test
public void test2(){
    /* My code */
}

@AfterClass
public void tearDown(){
    driver.quit();
}