@avondale is right on the mark here. The easiest way to do it is to edit the max distance for each pair of shoes. When I run longer ultras, I'll sometimes use 3 or 4 pairs of shoes in a single race, but if you add them to the run in Garmin, it adds the total mileage to each pair. To accommodate for that, I'll just add one pair to the run and edit its maximum mileage up by the amount run in the other shoes. For the other shoes, I'll subtract the mileage run in each pair from their maximum mileage, so everything nets out correctly.