It's not clear whether United were offered one of the duo or the choice between them, but this is a highly unexpected turn of events. As it's widely reported, the only two options are that it's completely true, or we're briefing something completely insane for some reason.
There have been reports that Mourinho wants both out, but we don't know if he has that much power at Chelsea. Neither appear to be out of favour at the club, so it's hard to see where the motivation for Chelsea to make such a crazy offer came from. Our rejection of it is slightly more understandable if it was Luiz who was offered, or simply because it was an opening bid that they could be persuaded to increase.
This is all very strange, and doubtless developments will be imminent. The prospect of Juan Mata and Shinji Kagawa lining up behind Robin van Persie next year would be quite something.