In a (large, recently constructed) garage, it seems likely that you've got multiple circuits with outlets. If you've got multiple outside outlets, they're probably all on the same circuit. If the hots are the same phase, I don't know if there's a way to determine if they're behind the same breaker or not. If the hots are on different phases, you need to return to each neutral or the gcfi that's likely to be present on an outdoor outlet will trip.