I guess we mean different things by 'moving'. They certainly moved from where they were in the first picture, which shows a fairly thinly populated area. My impression is that they hadn't yet chosen their vantage point, perhaps having just arrived. But in the later pictures, they appear to be standing still and watching the runners with everyone else. When I say they don't seem to be moving, I mean they don't appear to be moving relative to the crowd around them, eg departing the area.

I'm guessing they're not significant because while they first drew attention for both carrying heavy-looking bags (a black backpack and a blue duffel), that may be a subconscious bias towards the fact that there were two explosions. On the other hand, the two explosions were separated by a city block; so unless one or both of them were spotted a similar distance away from the start point, the fact of the two big bags is probably just coincidence.

So many people were taking photos and video that by the time the investigation is done, I imagine there'll be a patchy but contiguous animation of people's movements through the area, at which point it'll be far easier to spot dichotomies in behavior between race viewers and the criminal(s) that carried out the bombing.