The fix-a-flat will also eat away the rubber lining of the tire. This is a bad thing. I would never use this. A spare and a trip to have a patch/plug combo is always the best method (besides 2 new tires). The combo has the plug which you pull through the hole attached to a patch which further covers the hole. I have repaired many tires (100's) at the shop and seen many plugs leak, many patches leak, but never a combo. Of course, none of my patches returned, but other peoples did. =) Anyway the next wuestion is why 2 tires. If you replace 1 the tires will not be even side to side, which can cause a pull and further uneven tire wear. You did choose wisely to take it in and have them patch. I just hope they have a clue. Keep an eye on it for a while, first sign of loss take it back.