Multiple elapsed.time on a single event

Elastic Stack Logstash
1

//Below statement creates elapsed.time on end event of task1
elapsed {
start_tag => "task1Started"
end_tag => "task1Completed"
unique_id_field => "transactionId"
}
//Below statement creates elapsed.time on end event of task2
elapsed {
start_tag => "task2Started"
end_tag => "task2Completed"
unique_id_field => "transactionId"
}
//I would like add another elapsed.time on end event of task2 which task1 + task2. How to get this done?
elapsed {
start_tag => "task1Started"
end_tag => "task2Completed"
unique_id_field => "transactionId"
}

Appreciate any help.