Need help triggering a workflow in a Repeating Group when page loads

I’ve got a problem generating lists in a Repeating Group. This is something I’ve come across before but I can’t remember what the outcome was so I’m hoping someone can help me.

I’m using List Shifter to concatenate two lists and include the duplicates. That works and I’m using the resulting list as the Data Source for a repeating group. My problem is that I can only generate the list by triggering it with a button but I want the list to be visible in the repeating group On Page Load.
This is the button workflow that works…(not sure this image is much use!)

If I go to the Workflow tab and choose a New Workflow > On Page Load, the ListShifter element isn’t visible because it’s inside the repeating group.

I feel that I’m missing something really obvious here but I’m not sure what. Can anyone point me in the right direction?

You could use a “Do when condition is true” and select “Just once”. And make the condition be When RepeatingGroups List of Things :first item is empty. Then make sure the default data source is empty.