I cannot find a way to do this.
I have create a custom post type and use Simple Fields to assign this post type to multiple criteria.
Out of a list of 10, I'd like to be able to select 2 or 3 - but the radio buttons only allow me to select one. None of the other types let me select multiple options within the same field. Sure, I could use checkboxes - but then I have 10 fields, not 10 options to the same field.