Answer
When using Lightning Payroll desktop, if you already have an ATO machine credential file on your PC and need it on a second PC for STP, there are a couple of ways to do this.
The first way is to simply copy the credential file and move it to your new PC in two steps using Lightning Payroll's Tools menu at the top of the program:
1. Click Tools >> Export Machine Credential from your PC with the existing machine credential to save a copy of your ATO keystore XML to a USB or other location of your choice; click the blue folder on the right to choose where you will save the file. Please note: this will not remove your machine credential from this PC, it will just create a copy for your second PC, so that you can send STP from either one.
2. Then, click Tools >> Import Machine Credential >> blue folder from Lightning Payroll on your second PC to locate and load the machine credential XML file into this copy of Lightning Payroll desktop.
For example: export the machine credential on the existing STP payroll PC, saving it to a USB. Then transfer the USB to the second PC and use the Import Credential option to locate the XML file on the USB and import it for STP usage on PC #2. If you don't have a USB or similar to use, you may wish to attach the exported XML file to an email, and email it across to the second PC, where you can download the attachment from your email, and import it into your second PC.
The other option is to reapply for a new machine credential afresh from the ATO's RAM website.