in order for a machine to function as an Execution Server some sort of WebUI Test Studio instance will have to be installed. It doesn't matter whether it's QA,Dev or Run-Time edition.
Since you're running QA and Dev edition on another machine I recommend that you install Run-Time edition on the Execution Server.
Detailed documentation will be made available on Scheduling in the near future. Meanwhile please check out:
The Section on Scheduling in our Knowledge Base:
Video tutorials on Scheduling:
And this blog post on Scheduling:
Let me know if you have any other questions!
All the best,
the Telerik team