With the 6th edition rules, squads must all fire upon the same unit. An exception is made if the unit contains at least one model with the "split fire" rule (6ED, page 42).
Neither the Devastors, nor Tactical squads, in the Dark Angels codex (6ED, page 103), nor in the Blood Angels codex (5ED, page 95) have the "split fire" rule. As a side note, some Dark Angel units have it but no Blood Angels have it at all.
This youtube video (starting at 6:50) suggests that the squads have "Split Fire".
First question: where in the 6ED rules is it specified that Devastator squads have split fire?
Second question: do Tactical squads have split fire? If so, where in the 6ED rules is that specified?