IMO the brothers weren't seen walking together as one would expect because the FBI faked the video to separate them so it would look as if they were deliberately too far apart to be connected, pretending they didn't know each other and thus adding to their guilt as suspects.
The FBI is very good at faking evidence, having had years of practice.... as well at lying congenitally, organising ("facilitating", according to a federal judge quoted in the NY Times last April) the vast majority of terrorist attacks in the US, unlawful killing and a host of other crimes. Just ask its former agents, those who are still alive to talk.
