I think two bags would fit side by side nicely in a Costco cart, but I only brought one since I wasn't buying more than a bag full. I made sure to put all the UPCs facing up, they did the prescan thing at the register so I only had to scan my card when I got up to pay. I just rolled out to my car, picked up the bag, put the bag in my trunk and then did the same thing when I got home. I forget who suggested this trick here but it's great!
I'm definitely taking HF bags to Costco from now on. I think I'll try it at Aldi too, I'll bet my typical Aldi trip fits in one bag.
Also, HF is on the way to Costco so on my way I picked up some clamps at 30% off. On advice I saw here, I picked up a couple of the small Bremen clamps and I like them better than my DeWalts.