As I understand you set up two One Node Pools, and now you want to change one of the One node to a Execute node, I'm right?
Assuming I'm right:
1. You should change the DAEMONS in your condor_config file, and remove all but STARTD and MASTER
2. Change the CONDOR_HOST in the Execute node so it points to the Master node.
3. If you don't have a DNS, add in the /etc/hosts file of both nodes the FQDN of the Two nodes (so you can access by name to from one to another)
4. Restart Execute node.
I think with this you should get it working.
Good day :)