How to implement If Else block in Jmeter test plan?

I have a task where I need to perform two tasks based on the previous Sampler result. If Assertion fails { send email with the error message. }else { perform the tasks in while controller. }

I have achieved the first part easily by providing the condition in If controller ${JMeterThread.last_sample_ok} == false

I have problems going to else block. I have tried two IF controllers but with no success. Case here is need to perform one task only either IF block or Else block. But In my case it goes to both the blocks and perform both the tasks. Can anyone help how to achieve this?