It seems that each Fuel form can only have one set of tabs - defined using the 'fieldset' field type. Is that right?
In testing, my form had a group of fields nested inside fieldsets (class=tab), followed by a 'section', a few other fields and then another group of fields nested inside fieldsets (class=tab). It seems ALL the class=tab fieldsets get bunched together into a single tabset irrespective of where the fieldsets (class=tab) are positioned on the form.
It looks like it's the BaseFuelController.js code that's just looking for 'fieldset.tab' to group together.
It would be nice if we could set up fieldset groups on the form and have the BaseFuelController.js treat them as separate tabsets.