Hi Alex,
This has nothing to do with repeats. The checkboxes vs. radio buttons actually come from using select_one or select_multiple questions; select_multiples (as in the upper question in your screenshot) use checkboxes, while select_ones (as in the second question) use radio buttons.
If you want that second question to use checkboxes, you could make it a select_multiple, but that does mean people will be able to choose multiple answers. You could limit the constraint so that people can't submit when they select more than one answer, using a formula like count-selected(${multiple}) = 1 in the constraint column, but they'd still be able to select more than one on the interface, which could be confusing for people.
