Hi Penny,
Thank you for your question! It is definitely a great use case.
In general, we recommend that these types of questions be discussed with your Customer Success Manager since he/she would know the most about your business workflows and operations, but we can definitely make some recommendations.
As an alternative to using 2 tasks that represent the same task, you might consider using a validation rule that only allows resources to enter the maximum planned effort to capture actual effort in timesheets. Then you could use 2 custom fields. The first custom field would capture overtime (non-billable). The second custom field would equal "overtime (non-billable) + estimated effort" in order to see how many hours it actually took to complete the task.
You would need admin permissions to set this up.
For assistance with this customization, we offer a few solutions:
Reach out to your Customer Success Manager (let us know if you would like us to provide details)
Attend our twice-weekly Advanced Q&A for interactive answers to your questions - http://www.clarizen.com/resources/online-demo.html#second
Post to our Customizations Q&A forum - https://success.clarizen.com/forums/22175706-Customizations