I definitely lean towards them talking prior to the 911 call. Obviously, none of us knows what happened that night (much to my chagrin!) but there are too many oddities about the events, actions and conversations of that night for me to believe it all just fell into place the way it did. There are too many lies and inconsistencies for me to believe a lot of what they've both said. Sure, there's no doubt pieces of truth located in each statement... but not the whole truth by any stretch (IMO). And for whatever reason I have trouble making Darin involved in the actual killing, that only leads me to believe she did it but he helped cover it up. And while I don't think he was involved in the murders, I also don't happen to think he's got much depth to him which means he could have instantly chosen her and her freedom over any other choices he could have made that night. I think they were both young and incredibly shallow and had a "disposable society" mentality about them.
I could be totally wrong, and there could also be many other underlying factors involved (her having something to hold over his head, drugs, etc.)... but my bottom line is that they conspired together to cover up her crimes. There is just no other way I can justify in my mind that he didn't go near Damon that night, and nor did she.