Using the entity query API to identify entities with correlated values

The following code would achieve what described in the question.

$result = \Drupal::entityQuery('b')
  ->condition('b_a.entity:a.a_f1', 'v1')
  ->condition('b_a.entity:a.a_f2', 'v2')
  ->execute();

In the first string passed to condition():

• b_a is the field name
• .entity:a says that field is an entity reference for an a entity
• a_f1 and a_f2 are fields for the a entity

For more information about the values accepted for the first condition() argument, see QueryInterface::condition().

In the case the b_a field is a multi-value field, both the conditions would refer to the same delta, which means the value of the a_f1 and A_f2 fields are taken from the same a entity.
The code I wrote at the beginning of the answer returns all the entities for which both the conditions are met. If you want to get all the entities for which one of the conditions is true, the following code should be used.

$query = \Drupal::entityQuery('b');
$group = $query->orConditionGroup()
  ->condition('b_a.entity:a.a_f1', 'v1')
  ->condition('b_a.entity:a.a_f2', 'v2');
$result = $query->condition($group)->execute();