I would use a validation rule, this is a business rule and will throw a pop up if the criteria are not met. The solution is below for you.
Here is a link to business rule help if you every wish to know more about how they run: Using Business Rules
Thank you so much for you response.
I have created the Validation Rule and it still allows the requestor and owner to be the same once I save the change.
However if I change the expression to $(Owner_Valid == RequestorLink) then the message pops up, but regardless of what user I choose, it will still not allow me to save.
I'm not sure where I am going wrong?
can you paste in a screen shot?
Does $(Owner_Valid != RequestorLink_RecID) work as expected?
Thanks for the reply all. I managed to fix it. I was using "Owner" instead of "Owner_Valid" in my expression.
Thanks for the assistance