That's what I was thinking but then these were supposedly the 6 liter size which is one of the smaller sizes at about 6 lbs. Two of them filled with BBs or whatever would be a considerable amount of weight. Then he'd have to have two backpacks and maybe more weight like a change of clothes, etc. to throw off any suspicion just in case. Even carrying two backpacks wouldn't have caused anyone to suspect anything as he could have been holding them for two racers. I can see leaving them beside a trashcan but the act of putting a backpack inside a trashcan would likely be noticed. And to get by with that twice would be too risky but what do I know.
For all we know the packpacks were placed in the cans prior to the event, similar to the box cutters being on the planes at Logan Airport.
It could have been more than one person. The two devices were some 75 yds apart but then one person could have place both. We just don’t know enough at this stage to be sure.