How to Add More Computers to Workflow Farm
Install Workflow Manager 1.0 as explained in Installing Workflow Manager 1.0. After installing the server add the computer to the existing farm as explained in Joining an Existing Farm. If you had specified a certificate during creation of the farm, you will need to install the same certificate on the new computer joining the farm.
Workflow Manager only supports a farm with 1 computer or a farm with 3 computers.
Workflow Manager 1.0 MSDN Community Forum