Hello Anzal,
In order to accomplish the scenario that you are describing, you can do the following:
1. Create two target pools
2. Add each instance in one target pool
3. Make the "Slave" instance's target pool as a backup pool for the "Master" instance's target pool
4. Configure the 'Failover ratio' on your target pool.
5. Create a Forwarding rule that points to the "Master" target pool.
You can have more information about the forwarding rules and target pools in this
Help Center article.
I hope this helps.
Sincerely,
George