Thanks! That makes sense now. For the sake of others, this is the working conditional statement I’m using:
if ( (isset( $_POST['fields']['field_5259b4be85a66'] ) && !empty( $_POST['fields']['field_5259b4be85a66'] ))
||
(isset( $_POST['fields']['field_5259b44f99143'] ) && !empty( $_POST['fields']['field_5259b44f99143'] )) ){