A form can have multiple validation handlers. By default, all forms come with at least one validator, which is its own validateForm method. There is more that can be added. However, by default, the form will merely execute ::validateForm and all element validators. This allows you to invoke methods on other classes or other forms.

If a class provides method1 and method2, which it would like to execute as well, the following code can be added to the buildForm method:

$form_state->setValidateHandlers([ 
  ['::validateForm'], 
  ['::method1'], 
  [$this, 'method2'], 
]); 

This sets the validator array to execute the default validateForm method and the two additional methods. You can reference a method in the current class using two colons (::) and the method name. Alternatively, you can use an array that consists of a class instance and the method to be invoked.